Pyrazolylimine iron and cobalt, and pyrazolylamine nickel complexes: Synthesis and evaluation of nickel complexes as ethylene oligomerization catalysts

Article abstract of DOI:10.1016/j.poly.2013.01.018

A series of tridentate (O^N^N) and bidentate (N^N) iron, cobalt and nickel complexes were prepared from {2,4-di-tert-butyl-6-[(2-(3,5-RR-pyrazol-1-yl- ethylimino)methyl]-phenol}, R = H (L1), Me (L2), Ph (L3), [(3,5-di-tert-butyl-2- ethoxy-benzylidene)-[2-(3,5-dimethyl-pyrazol-1-yl)-ethyl]amine (L4), and (4-(4-tert-butyl-benzylidene)-[2-(3,5-dimethyl-pyrazol-1-yl)ethyl]-imine) (L5). Ligands L1-L5 were reacted with MX₂ (M = Fe, Co, Ni; X = Cl, Br) to form three types of complexes. Whereas tridentate O∧N∧N bound complexes were formed with Fe(II) and Co(II) salts (1-6), reactions involving Ni(II) salts resulted in hydrolysis of the Schiff base ligands used to form a novel tetranuclear cubane-like cluster [Ni₄(3,5-Me₂pza) ₄Cl₈] (7) (pza = (3,5-dimethylpyrazol-1yl)ethylamine) and mononuclear [Ni(3,5-R₂pza)Br₂] (R = H (8); R = Me (9) complexes. Molecular structures of five complexes (1, 5-7 and 9), determined by single-crystal X-ray diffraction, confirmed that ligand hydrolysis occurred in the nickel reactions. All three nickel complexes showed good to excellent activity towards ethylene oligomerization with activities up to 10 726 kg (mol Ni h)^-1.

Full text of DOI:10.1016/j.poly.2013.01.018

Polyhedron 53 (2013) 295–303
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Polyhedron
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Pyrazolylimine iron and cobalt, and pyrazolylamine nickel complexes: Synthesis
and evaluation of nickel complexes as ethylene oligomerization catalysts
Michael K. Ainooson a, Ilia A. Guzei a,b, Lara C. Spencer b, James Darkwa a,
⇑
a Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, South Africa
b Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of tridentate (O^N^N) and bidentate (N^N) iron, cobalt and nickel complexes were prepared from
{2,4-di-tert-butyl-6-[(2-(3,5-RR-pyrazol-1-yl-ethylimino)methyl]-phenol}, R = H (L1), Me (L2), Ph (L3),
[(3,5-di-tert-butyl-2-ethoxy-benzylidene)-[2-(3,5-dimethyl-pyrazol-1-yl)-ethyl]amine (L4), and (4-(4-
tert-butyl-benzylidene)-[2-(3,5-dimethyl-pyrazol-1-yl)ethyl]-imine) (L5). Ligands L1–L5 were reacted
with MX2 (M = Fe, Co, Ni; X = Cl, Br) to form three types of complexes. Whereas tridentate O^N^N bound
complexes were formed with Fe(II) and Co(II) salts (1–6), reactions involving Ni(II) salts resulted in
hydrolysis of the Schiff base ligands used to form a novel tetranuclear cubane-like cluster [Ni4(3,5-
Me2pza)4Cl8] (7) (pza = (3,5-dimethylpyrazol-1yl)ethylamine) and mononuclear [Ni(3,5-R2pza)Br2]
(R = H (8); R = Me (9) complexes. Molecular structures of five complexes (1, 5–7 and 9), determined by
single-crystal X-ray diffraction, confirmed that ligand hydrolysis occurred in the nickel reactions. All
three nickel complexes showed good to excellent activity towards ethylene oligomerization with
Received 3 September 2012
Accepted 22 January 2013
Available online 31 January 2013
Keywords:
Iron
Cobalt
Nickel
Complexes
Imine hydrolysis
Ethylene oligomerization
Catalysts
activities up to 10726 kg (mol Ni h)À1
.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
by Kelly et al. [9], demonstrates the possibility to achieve enhanced
rate (2 Â 104) of hydrolysis of urethane by using metal-bound
water or hydroxide in the facile metal-assisted hydrolysis of ure-
thane. Also Tanaka and co-workers [10] have reported the use of
cyclic imines in their reaction with phenol in water over a wide
pH range to give a Mannich-type products. The kinetics involving
the hydrolysis of imines has been extensively studied by Jencks
et al. [11]. Although metals have been known in some instances
to stabilize Schiff base ligands, such kinetic study have implicated
metals as the Lewis acids responsible for the hydrolysis of these li-
gands. For example metals are reported to promote C–N hydrolysis
in azo-methine ligands and several hydrolysis of imines that are
reported in the literature [12]. Recently Chattopadhyay et al. [13]
have even shown the effect of counter ions on the hydrolysis of
imine bonds. It is therefore clear that several factors can affect
the successful use of Schiff base compounds as ligands in metal
complexes.
We report the preparation of new complexes containing
Schiff base ligands of the type 2,4-di-tert-butyl-6-[(2-pyrazol-1-yl-
ethylimino)methyl]phenol (L1), 2,4-di-tert-butyl-6-[(2-(3,5-dim-
ethylpyrazol-1-yl-ethylimino)methyl]phenol (L2) and 2,4-di-tert-
butyl-6-[(2-(3,5-diphenylpyrazol-1-yl-ethylimino)methyl]phenol
(L3), 4-tert-butyl-benzylidene-[2-(3,5-dimethyl-pyrazol-1-yl)ethy-
l]imine (L4) and (3,5-di-tert-butyl-2-phenoxy-benzylidene-(2-pyr-
azol-1-yl-ethyl)imine (L5) with MX2 (M = Fe, Co, Ni; X = Cl, Br).
Whereas the iron(II) and cobalt(II) complexes are generally stable
in solution and give mostly tridentate O^N^N bound octahedral or
Schiff base ligands have been synthesized and used in number
of catalytic reactions [1]. Their popularity stems from their ease
of synthesis, their ability to stabilize metals in different oxidation
states and the ability to modify both the electronic and steric prop-
erties of the ligands. In particular the salicylaldiminato motif has
the ability to coordinate metals through hard nitrogen and oxygen
donor atoms, which leads to better stabilization of metal com-
plexes against reduction and confers unusual thermal stability
[2]. In the area of catalysis Schiff base ligands have been used
extensively in ethylene oligomerization and polymerization reac-
tions [1,3], and other C–C transformation reactions [4].
Despite the advantages offered by Schiff base metal complexes,
hydrolysis of the imine group in the ligand is a major drawback in
the use of Schiff base ligands. Cyclic Schiff base ligands are gener-
ally known to be more stable in aqueous solution compared to the
non-cyclic ones. Also the stability of metal ion-imine bonds is more
evident in macrocyclic metal complexes where the metal–ligand
bond breaks considerably more slowly than in the corresponding
open-chain complexes [5]. In spite of hydrolysis being a problem
in Schiff base metal complexes, there are examples of metal as-
sisted hydrolysis that have been used in organic processes such
as hydrolysis of phosphates [6], amides [7] and acetals [8]. A report
⇑
Corresponding author. Fax: +44 1223 336 033.
E-mail address: jdarkwa@uj.ac.za (J. Darkwa).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.018

296
M.K. Ainooson et al. / Polyhedron 53 (2013) 295–303
tetrahedral complexes, the nickel(II) complexes are extremely sus-
ceptible to hydrolysis. Although the formation of hydrolysed nicke-
l(II) compounds were not the desired results, the hydrolysed nickel
products proved to be highly active oligomerization catalysts for
ethylene to 1-butenes and 1-hexenes.
(m, 5H, Ar-H); 8.07 (s, 1H, CH@N). IR (Diamond ATR, cmÀ1):
1623 (CH@N).
t
2.3. Synthesis of metal complexes
2.3.1. Bis[{2,4-di-tert-butyl-6-(2-(pyrazol-1-
yl)ethylimino)}phenolateFe(II)][tetrachloro-ferrate(II)] (1)
2. Experimental
A solution of L1 (0.26 g, 0.80 mmol) in CH2Cl2 (10 mL) was
added dropwise to a suspension of FeCl2 (0.10 g, 0.80 mmol) in
CH2Cl2 (15 mL) with constant stirring. A dark green suspension be-
gan to form after 2 min and the reaction to stirred at room temper-
ature for 18 h, leading to a dark green solution. After filtration, the
resultant solution was concentrated to about 10 mL, gently layered
with hexane (5 mL) and stored at À4 °C to precipitate a dark green
powder. The powder was isolated by filtration, washed with hex-
ane (2 Â 5 mL) and the product dried in air. Yield: 0.53 g (71%).
Anal. Calc. for C40H56Cl4N6O4Fe2: C, 51.20; H, 6.01; N, 8.96. Found:
C, 51.10; H, 6.29; N, 8.66%. IR (Diamond ATR, cmÀ1): 1600
2.1. Materials and instrumentation
All experiments were carried out in a dry nitrogen atmosphere.
All solvents were dried and distilled prior to use. Pyrazole, 3,5-
dimethylpyrazole, FeCl2, CoCl2, NiCl2Á6H2O and [NiBr2(DME)] were
purchased from Sigma–Aldrich and used as received. The ligands:
2,4-di-tert-butyl-6-[(2-pyrazol-1-yl-ethylimino)methyl]phenol (L1),
2,4-di-tert-butyl-6-[(2-(3,5-dimethylpyrazol-1-yl-ethylimino)methyl]
phenol (L2) and 2,4-di-tert-butyl-6-[(2-(3,5-diphenylpyrazol-1-
yl-ethylimino)methyl]phenol (L3), were synthesised using a proce-
dure reported by Boltina et al. [14].
t(CH@N).
Complexes 2–6 were prepared in a similar manner, using the
IR spectra were recorded on a Bruker Tensor 25 equipped with a
diamond ATR. NMR spectra were recorded on a Varian Gemini
2000 instrument (1H at 300 MHz and 13C at 75 MHz). Chemical
shifts are reported in d (ppm) and referenced to the appropriate
peak in CDCl3 used as NMR solvent. Mass spectra were run by
the Analytical Services Unit at the University of Stellenbosch
(South Africa) on a Waters API Quattro micro. Elemental analyses
were performed on a Vario elementar III microcube CHNS analyser
by the Department of Chemistry, Rhodes University (South Africa).
Gas chromatography (GC) analyses were conducted on a Varian
3900 GC equipped with an FID detector and a Factor Four Capillary
Column (VF-1ms 15 M Â 0.25 MM ID DF = 0.25).
appropriate ligand and metal salt.
2.3.2. {2,4-Di-tert-butyl-6-(2-(3,5-dimethylpyrazol-1-
yl)ethylimino)}methylphenolateFeCl (2)
The reaction of L2 (0.20 g, 0.56 mmol) and FeCl2 (0.07 g,
0.56 mmol) gave a dark green solid. Yield: 0.22 g (88%). Anal. Calc.
for C22H32ClN3OFeÁ0.5 CH2Cl2: C, 55.35; H, 6.81; N, 8.61. Found: C,
55.46; H, 6.87; N, 7.14%. IR (Diamond ATR, cmÀ1): 1609
t(CH@N).
ESI-MS (m/z) (%) = 479 (50%) [M]+, 445 (50%) [MÀCl]+.
2.3.3. {2,4-Di-tert-butyl-6-(2-(3,5-diphenylpyrazol-1-
yl)ethylimino)}methylphenolateFeCl (3)
The reaction of L3 (0.20 g, 0.41 mmol) and FeCl2 (0.053 g,
0.42 mmol) gave a fluffy dark green solid. Yield: 0.18 g (75%). Anal.
Calc. for C32H38ClN3OFe: C, 67.20; H, 6.70; N, 7.34. Found: C, 67.57;
2.2. Synthesis of ligands
H, 6.35; N, 7.00%. IR (Diamond ATR, cmÀ1): 2980
t(C–H), 1609
2.2.1. [2-(3,5-Dimethyl-pyrazol-1-ylethyl)]-(3-methyl-
benzylidene)imine(L4)
t
(CH@N). ESI-MS (m/z) (%) = 570 (5%) [M]+, 534 (5%) [MÀCl]+.
A solution of 4-tert butylbenzaldehyde (1.20 g, 7.30 mmol) and
2-(3,5-dimethyl-pyrazol-1-yl)ethylamine (1.00 g, 7.20 mmol) in
ethanol (30 mL) was refluxed for 18 h. After evaporating the sol-
vent a viscous yellow oil, which later solidified to a bright yellow
crystalline mass upon leaving the oil at room temperature over-
night, was obtained. Pure L4 was isolated after recrystallization
from a 1:1 mixture of CH2Cl2 and hexane. Yield: 1.90 g (94%).
HRMS (ESI) m/z [M+H]+: Anal. Calc. for C18H27N3 284.2117. Found:
2.3.4. {2,4-Di-tert-butyl-6-(2-pyrazol-1-
yl)ethylimino}methylphenolateCoCl (4)
Ligand L1 (0.90 g, 2.7 mmol) and anhydrous CoCl2 (0.34 g,
2.6 mmol) gave a green powder. Yield: 0.70 g (64%). Anal. Calc.
for C20H28ClN3OCo: C, 57.08; H, 6.70; N, 9.88. Found: C, 57.24; H,
6.89; N, 10.31%. IR (Diamond ATR, cmÀ1), 1623
t(CH@N). ESI-MS
(m/z) (%) = 711 (100%) [(L1)2Co]+; 444 (80%) [M+Na]+; 420 (10%)
[M]+.
284.2119. IR (Diamond ATR, cmÀ1): 1648
t
(CH@N). 1H NMR
(CDCl3): d 1.29 (s, 9H, t-Bu); 2.14 (s, 3H, CH3); 2.18 (s, 3H, CH3);
2.3.5. {2,4-Di-tert-butyl-6-(2-(3,5-dimethylpyrazol-1-
yl)ethylimino)}methylphenolateCoCl (5)
3
3
3.94 (t, 2H, JHH = 4.5 Hz, CH2); 4.26 (t, 2H, JHH = 4.5 Hz, CH2);
3
5.64 (s, 1H, pz-H); 7.37 (d, 1H, JHH = 8.4 Hz, Ar-H); 7.55 (d, 1H,
Ligand L2 (0.22 g, 0.62 mmol) anhydrous CoCl2 (0.083 g,
0.66 mmol) gave a green powder. Yield: 0.23 g (78%). Anal. Calc.
for C22H32ClN3OCo: C, 58.87; H, 7.19; N, 9.36. Found: C, 58.32; H,
2JHH = 8.4 Hz, Ar-H); 7.91 (s, 1H, CH@N). 13C{1H} NMR (CDCl3): d
11.1; 13.5; 31.1; 34.8; 48.8; 61.4; 104.4; 125.5; 127.7 133.1;
139.8; 147.5; 154.1; 163.0.
7.65; N, 9.69%. IR (Diamond ATR, cmÀ1): 2990
t(OH) 1609
t
(CH@N). ESI-MS (m/z) (%) = 478 (5%) [M+Na]+.
2.2.2. 3,5-Di-tert-butyl-2-phenoxy-benzylidene)-(2-pyrazol-1-
ylethyl)imine (L5)
2.3.6. {2,4-Di-tert-butyl-6-(2-(3,5-diphenylpyrazol-1-
An EtOH (15 mL) solution of 3,5-di-tert-butyl-2-phenoxybenz-
aldehyde (1.2 g, 3.8 mmol) was added to pyrazolyl-1-yl-ethyla-
mine (0.65 g, 5.9 mmol) in EtOH (15 mL). Anhydrous magnesium
sulfate (1.0 g) was added to this solution and refluxed for 4 h. After
filtration, the solvent was evaporated to give a yellow crystalline
solid which was recrystallized from 1:1 mixture of CH2Cl2 and hex-
ane. Yield: 0.81 g (65%). HRMS (ESI) (m/z) [M+H]+: Anal. Calc. for
yl)ethylimino)}methylphenolateCoCl (6)
Ligand L3 (0.14 g, 3.0 mmol) and anhydrous CoCl2 (0.14 g,
3.0 mmol) gave dark blue crystals. Yield: 0.11 g (62%). Anal. Calc.
for C32H36ClN3OCo: C, 67.07; H, 6.33; N, 7.33. Found: C, 67.30; H,
6.67; N, 7.22%. IR (Diamond ATR, cmÀ1): 1617
t(CH@N).
2.3.7. {2-(3,5-Dimethylpyrazol-1yl)ethylamino}NiCl2 (7)
C
26H33N3O: 404.2654. Found: 404.2659. 1H (CDCl3): d 1.17 (s, 9H,
A mixture of L4 (0.80 g, 2.8 mmol) and NiCl2Á6H2O (0.67 g,
2.8 mmol) in THF (20 mL) was refluxed for 18 h. The resultant
mixture was filtered and the filtrate evaporated to give a green
3
t-Bu); 1.40 (s, 9H, t-Bu); 3.98 (t, 2H, JHH = 5.7 Hz, CH2); 4.44 (t,
3
3
2H, JHH = 5.7 Hz,CH2); 6.23 (t, 1H, JHH = 1.8 Hz, pz-H); 7.35–7.69

M.K. Ainooson et al. / Polyhedron 53 (2013) 295–303
297
powder. The product was recrystallised from CH2Cl2:hexane. Yield:
0.56 g (75%). Anal. Calc. for C7H13Cl2N3Ni: C, 31.28; H, 4.87;
N, 15.63. Found: C, 31.66; H, 4.89; N, 15.31%. IR (Diamond
the oligomerization reaction. A toluene (80 mL) solution of the cat-
alyst precursor was first introduced into the reactor and the solu-
tion saturated with ethylene at 1 atm. Ethylaluminium dichloride
was then injected into the reactor as a co-catalyst. The desired eth-
ylene pressure was set and the oligomerization reaction run at a
set temperature and time. The reaction was terminated by turning
off the ethylene feed and reactor slowly vented of volatile contents.
The vented volatiles were trapped and sampled for the presence of
ethylene oligomers. The contents in the reactor were quenched
with 10% HCl in methanol and the product sampled for GC analysis.
ATR, cmÀ1): 1615
t(CH@N). ESI-MS(+) (m/z) (%) = 780 (70%)
[{Ni4Cl7(3,5Me2pza)2}Na]+, 510 (100%).
2.3.8. {2-Pyrazol-1-yl)ethylamino}NiBr2 (8)
A solution of L5 (0.44 g, 1.1 mmol) in THF (20 mL) was added
dropwise to a solution of [NiBr2(DME)] (0.34 g, 1.1 mmol) in THF
(10 mL). An intense green solution was formed after 20 min which
was stirred for 18 h at room temperature. After removal of the sol-
vent the residue was recrystallised from CH2Cl2 and hexane. Yield:
0.43 g (63%). Anal. Calc. for C5H9Br2N3Ni: C, 18.38; H, 2.16; N,
12.75. Found: C, 18.28; H, 2.07; N, 12.65%. IR (Diamond ATR,
3. Results and discussion
cmÀ1): 3280
(100%) [M+THF]+.
t
(N–H), 1617
t(CH@N). ESI-MS (m/z) (%) =695
3.1. Synthesis of ligands and metal complexes
Ligands L1–L3 were synthesized by the method recently re-
ported by us [14] while L4 and L5 were prepared using this meth-
od. Both L4 and L5 were isolated as yellow solids and exhibited
characteristic imine peaks (L4 (7.91 ppm) and L5 (8.07 ppm)) and
two sets CH2 triplets for L4 and L5 in the range 3.97–4.44 ppm in
their 1H NMR spectra; similar to those observed for L1–L3. The
imine and CH2 peaks were observed in addition to other aliphatic
and aromatic protons that are unique to L4 and L5.
The reations of L1–L5 with FeCl2, CoCl2, NiCl2 and NiBr2 gave
three different types of products (Schemes 1 and 2). The first prod-
uct type, complex 1, was obtained only with L1 and FeCl2
(Scheme 1). Iron(II) chloride reacted with L1 to give HCl as a by-
product; similar to the reactions of L1–L3 with PdCl2 [14]. This
was followed by the reaction of a second equivalent of L1 with
an interemediate iron product to form the cationic species found
in 1. The [FeCl4]2À counterion in 1 is likely to be the result of two
equivalents of HCl reacting with a second equivalent of FeCl2. Com-
plex 1 was characterized by a combination of IR spectroscopy, ele-
mental analysis and single crystal X-ray crystallography (vide
supra). One of the key spectroscopic data in the characterisation
of 1 is IR spectrum. The CH@N peak at 1613 cmÀ1 for L1 shifted
to 1620 cmÀ1 in 1, indicative of coordination of L1 to the Fe and
also that the imine functionality in 1 did not hydrolyse.
2.3.9. {2-(3,5-Dimethylpyrazol-1yl)ethylamino}NiBr2 (9)
Complex 9 was prepared in similar manner as described for 7,
using L4 (0.33 g, 1.20 mmol) and NiBr2 (0.026 g, 1.20 mmol). The
purple powder obtained was washed 3 Â 5 mL hexane and dried
under vacuum. Crystals suitable for X-ray crystallography were
grown from a THF solution of 9, kept at À4 °C for 3 days. Yield:
0.60 g (72%). Anal. Calc. for C7H13Br2N3NiÁ2THF: C, 35.89; H, 5.82;
N, 8.37. Found: C, 35.01; H, 5.68; N, 9.13%. IR (Diamond ATR,
cmÀ1): 3300
[M+Na]+.
t(NH2), 1609 t(CH@N). ESI-MS (m/z) (%) =522 (50%)
2.4. Crystal structure determination
Crystallographic data were collected on a Bruker SMART APEXII
diffractometer with CuÁK (k = 1.54178 Å). In a typical experiment a
a
crystal was selected under oil under ambient conditions and at-
tached to the tip of a MiTeGen MicroMountÓ. The crystal was
mounted in a stream of cold nitrogen at 100(1) K and centred in
the X-ray beam using a video camera. The initial cell constant
was obtained from three series of
gles. Each series consisted of 50 frames collected at intervals of 0.5°
in a 25° range about with the exposure time of 5 s per frame. The
x scans at different starting an-
x
reflections were successfully indexed by an automated indexing
routine built in the SMART program. The final cell constants were
calculated from a set of strong reflections from the actual data
collection.
The second reaction type involves L2 and L3 with FeCl2 and L1–
L3 with CoCl2 to produce pincer-type complexes of general formula
[(O^N^N)MCl] (M = Fe, Co) (2–6), in which HCl was formed as a by-
product, but only one equivalent of ligand reacted with the metal
chloride (Scheme 1). This second reaction type suggests that the
formation of complex 1 is driven by steric factors where the larger
size of Fe2+, compared to that of Co2+, allows for a second molecule
of L1 to coordinate to the Fe; whereas the smaller Co2+ does not al-
low coordination of a second equivalent of L1. Similar steric con-
sideration would explain why L2 and L3 do not form complexes
of similar composition as 1 when reacted with FeCl2.
The data were collected by using the full sphere data collection
routine to survey the reciprocal space to the extent of a full sphere
to a resolution of 0.82 Å. Highly redundant datasets were corrected
for Lorentz and polarization effects. Absorption corrections are
based on fitting a function to the empirical transmission surface
as sampled by multiple equivalent measurements. Upon the anal-
ysis of the systematic absences in the diffraction data the structure
were solved in the correct space group to obtain chemically rea-
sonable and computationally stable results of refinement [15–
17]. A successful solution by the direct methods provided most
non-hydrogen atoms from the E-map. The remaining non-hydro-
gen atoms were located in an alternating series of least-squares cy-
cles and difference Fourier maps. All non-hydrogen atoms were
refined with anisotropic displacement coefficients. All hydrogen
atoms were included in the structure factor calculation at idealized
positions and were allowed to ride on the neighbouring atoms with
relative isotropic displacement coefficients.
Complexes 2–6 showed 1H NMR spectra with chemical shifts
ranging from À20 to 80 ppm, confirming the paramagnetic nature
of these Fe and Co pincer complexes. These complexes were there-
fore characterised by a combination of IR spectroscopy, mass spec-
trometry, elemental analyses; and for
5
and
6
further
characterisation by single crystal X-ray crystallography. Again IR
peaks in the range 1609–1623 cmÀ1 were indicative of non-hydro-
lysed CH@N functional groups in all five complexes, while molec-
ular ions in the mass spectra suggested these complexes conform
to the formula [(O^N^N)MCl].
The third reaction type is nickel induced hydrolysis of the Schiff
base ligands L1–L5 when these ligands were reacted with nickel(II)
dihalides. This reaction is typified by L2, L4 and L5 (Scheme 2),
leading to the formation of (pyrazol-1-ylethyl)amine nickel(II)
dihalides (7–9). Initially we thought the reactions where ligands
L1–L3 hydrolysed were facilitated by the HCl produced in reactions
2.5. Ethylene oligomerization reactions
In a typical procedure for the oligomerization of ethylene, a
500 mL steel autoclave was heated under vacuum for 2 h at
160 °C and subsequently cooled to the temperature required for

298
M.K. Ainooson et al. / Polyhedron 53 (2013) 295–303
tBu
R
2+
N
Fe
N
Cl
But
O
N
O
N
O
M
2-
N
FeCl4
N
N
R
O
N
2 FeCl2
= L1-;
MCl2
R = H, (1)
R = Me; M = Fe (2)
R = Ph; M = Fe (3)
R = H, M = Co (4)
R = Me, M = Co (5)
R = Ph; M = Co (6)
tBu
N
R
But
OH
N
N
N
R
(R = H (L1); Me (L2); Ph (L3))
Scheme 1. Reaction scheme for the synthesis of complexes 1–6.
tBu
Me
Me
But
OH
But
N
N
N
N
N
N
Me
(L2)
Me
NiX2
(L4)
R
NiX2
X
X
N
Ni
H2N
N
R = Me; X = Cl (7)
R = H, X = Br (8)
R = Me, X = Br (9)
R
NiX2
tBu
But
OPh
N
N
N
(L5)
Scheme 2. Reaction scheme for the synthesis of complexes 7–9.
involving these ligands and nickel(II) dihalides. We therefore pro-
ceeded to use a non-phenolic ligand (L4) and also to functionalise
the phenolic OH group in L1 to a phenoxy group (L5) to avoid the
formation of HCl. However, when L4 and L5 were reacted with
nickel(II) dihalides this did not stop the hydrolysis of L4 and L5,
thus confirming that the hydrolysis is not induced by HCl but the
acidic nature of the nickel(II) salts. There are several examples of
metal catalysed hydrolysis of Schiff base nickel [18] and copper
[19] complexes, including a recent report by us on the hydrolysis
of pyrazol-1-ylimines [20]. In all these hydrolyses reactions it is
clear that even traces of water triggers the hydrolysis because care-
ful attempts to exclude water still gave small amounts of the
hydrolysis products (7–9); hence in preparing 7–9 as described
in the experimental section, we took no precautions to exclude
water.
Two things were clear from the characterisation of complexes
7–9. First, all three complexes were extremely hygroscopic and
for complex 9 elemental analysis only made sense after the crystal

M.K. Ainooson et al. / Polyhedron 53 (2013) 295–303
299
MA259
MS_Direct_091009_4 16 (0.177) Cn (Cen,4, 80.00, Ar); Sm (SG, 1x5.00); Sb (1,40.00 ); Cm (16:22-2:7)
TOF MS ES+
3.79e3
501.0
100
N
Cl
Cl
Ni
N*
Cl
503.0
511.0
Ni
Cl
780.0
Cl
Ni
Cl
Na
499.0
Ni
N
N*
Cl
782.0
513.0
770.0
768.0
784.0
515.0
515.2
766.0
760
786.0
788.0
497.0
421.8
420
434.8
549.7
452.8
460
541.2
540
475.7
0
m/z
840
440
480
500
520
560
580
600
620
640
660
680
700
720
740
780
800
820
Fig. 1. ESI-MS of tetranuclear nickel cluster 7
structure of this complex was obtained, showing that 9 crystallises
with three water molecules in the coordination sphere (vide supra).
Secondly, the basic unit in 7 self-assembled into a cubane-like clus-
ter via hydrogen bonding between a chloride of one molecule and
the hydrogen in the N–H unit of an adjacent molecule (Fig. S2). The
self-assembly was evident in the ESI-MS (Fig. 1) where one of the
observed ions at m/z = 780 could be assigned to [{Ni4Cl7(3,5-
Me2pza)2}Na]+.
Complexes 7–9 are unique in that pyrazolylethylamine ligands
either self-assemble (7) or leads to a change in the coordination
environment where a halide is replaced by water in the coordina-
tion sphere (9). This provides an organized synthesis of the cub-
ane-cluster (7) or an avenue to controlled hydrolysis involving
the halide (8).
modelled by finding four positions for each of the three remaining
Cl atoms Cl6, Cl7, and Cl8. The relative contributions of the four
disorder components refined with similarity restraints are
50.0(5)%, 18.8(4)%, 23.0(3)%, and 8.2(3)%. There is also one disor-
dered partially occupied molecule of dichloromethane in the asym-
metric unit. Two disorder components were refined isotropically
with a rigid DFT-calculated geometry with occupancy factors of
55.0(10)% and 19.9(8)%. Possibly, there are also two water mole-
cules present in the asymmetric unit, each with a 16(2)% occu-
pancy. No hydrogen atoms were assigned to these water
molecules.
The coordination environment about the Fe(II) centers in the
cations is a distorted octahedron. The overall geometries of the
two cations are similar. The Fe centers bear two tridentate
O^N^N chelates with the two oxygen atoms positioned cis to each
other. The cis atom-Fe-atom angles vary between 83° and 99°, indi-
cating a rather substantial deviation from the ideal geometry. The
Fe–O (av. 1.890(10) Å) and the Fe–N (av. 2.16(5) Å) bond distances
are comparable to bond lengths observed in related iron complexes
[21].
Complex 5 crystallizes with two symmetry independent mole-
cules in the asymmetric unit. The two molecules are enantiomers,
with the cobalt metal acting as a chiral center. Moreover, the crys-
tal of 5 is a racemic twin with a 53.5(2):46.5(2) twin component
ratio. The ligand acts as a tridentate chelate, forming two six mem-
bered metallacyclic rings. The Co is in a flattened tetrahedral envi-
ronment formed by two N atoms, a Cl atom, and an O atom. The
Co(1)–N(1), Co(1)–N(3), Co(1)–O(1) bond distances for the two
molecules average 2.026(3) Å, 1.974(3) Å, and 1.884(5) Å, respec-
tively, whereas the bond angles about the Co centres vary from
94.02(11)° to 127.65(8)° (Fig. 3).
3.2. Molecular structures of complexes 1, 5–7 and 9
Although elemental analysis and mass spectral data pointed to
possible structures for all nine complexes, it was not until after the
crystal structures of these five complexes were determined that
the elemental analyses could be interpreted with confidence. The
results of five single-crystal diffraction experiments unequivocally
established the solid state compositions of the five selected com-
pounds. Crystal data collection and refinement data for these com-
plexes are in Table 1 and the structures are depicted in Figs. 2–6.
The crystal structure of 1 contains two ionic pairs of Fe com-
plexes and solvent molecules as follows. There are two symmetry
independent cations with atoms Fe1 and Fe2 in the structure. In
the Fe2 cation one tert-butyl group is disordered over two posi-
tions with the major component contribution of 57.5(3)%. The dis-
ordered atoms were refined isotropically. The two disorder
components were refined with similarity restraints. There are also
two anions with atoms Fe3 and Fe4 in the structure. The Fe4 anion
is disordered by rotation about the Fe4–Cl5 axis. The disorder was
Complex 6 co-crystallises with one equivalent of solvent mole-
cule of acetone. The solid state geometry of complex 6 is similar to
that of 5 and has a four-coordinate Co center with two Co–N bonds

300
M.K. Ainooson et al. / Polyhedron 53 (2013) 295–303
Table 1
Crystallographic data for complexes 1, 5, 6, 7 and 9.
1
5
6
7
9
Empirical formula
[Fe(C20H28N3O2)2]2+[FeCl4]2À C22H32ClCoN3O
.0.75CH2Cl2.0.32H2O
C32H36ClCoN3OÁC3H6O C28H52Cl8N12Ni4
[C7H19BrN3NiO3]2+2BrÀ.H2O
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
935.95
100(2)
0.71073
monoclinic
P21/n
448.89
100(2)
1.54178
orthorhombic
Pca21
631.10
100(2)
0.71073
monoclinic
P21/n
1075.26
100(2)
0.71073
rhombohedral
855.8
100(2)
0.71073
monoclinic
P21/n
ꢀ
R3
17.305(3)
20.444(4)
26.922(5)
90
16.424(15)
7.805(3)
35.561(7)
90
15.268(2)
14.742(2)
15.731(2)
90
36.596(10)
36.596(10)
23.676(6)
90
13.4870(15)
8.6652(9)
24.930(3)
90
b (Å)
c (Å)
c
(°)
b (°)
95.408(3)
90
9482(3)
8
1.311
0.909
3909
90
90
4558(2)
8
1.308
7.101
1896
112.897
90
3261.8(9)
4
1.285
0.643
90
120
27460(13)
18
1.170
92.782(2)
90
2910.1(5)
4
1.921
6.827
c
(°)
Volume (Å3)
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmÀ1
F(000)
)
1.592
9936
1332
1672
Crystal size (mm3)
0.42 Â 0.28 Â 0.18
0.38 Â 0.33 Â 0.30
0.45 Â 0.40 Â 0.40
0.36 Â 0.34 Â 0.19 0.51 Â 0.45 Â 0.42
Theta range for data collection (°)
Index ranges
1.25–25.00
2.48–69.77
2.38–7.50
1.11–25.11
À43 6 h 6 43,
À43 6 k 6 43,
28 6 l 6 28
93464
2.49–28.29
À20 6 h 6 20, À24 6
k 6 24, À32 < =l <= 31
À18 6 h 6 16,À9 6 À19 6 h 6 19,
À17 6 h 6 17,
k 6 9,À38 6 l 6 43
À19 6 k 6 19,
20 6 l 6 20
28018
À11 6 k 6 11, À33 6 l 6 33
Reflections collected
98256
42533
43979
Independent reflections
16621 [R(int) = 0.0527]
7904
7424 [R(int) = 0.0462] 10787
[R(int) = 0.0618]
99.6%
7197 [R(int) = 0.0325]
[R(int) = 0.0296]
97.8%(h = 69.77°)
empirical with
SADABS
Completeness to theta %
Absorption correction
99.5 %
empirical with SADABS
99.1%
99.7%
multi-scan with
SADABS
multi-scan with
SADABS
multi-scan with SADABS
Max. and min. transmission
0.8534 and 0.7013
0.2245 and 0.1733 0.7831 and 0.7608
0.7518 and
0.5979
full-matrix least-
squares on
10787/0/477
0.967
0.1616 and 0.1284
Refinement method (F2)
full-matrix least-squares on
full-matrix least-
squares on
7904/1/522
1.011
full-matrix least-
squares on
7424/0/387
1.107
full-matrix least-squares on
Data/restraints/parameters
Goodness-of-fit on F2
16621/47/1010
1.106
7197/0/370
1.021
Final R indices [I > 2sigma(I)]
R1 = 0.0975, wR2 = 0.2340
R1 = 0.0274, wR2
0.0653
R1 = 0.0307, wR2
0.0668
=
R1 = 0.0476,
wR2 = 0.1193
R1 = 0.0730,
wR2 = 0.1264
0.591 and À0.425
R1 = 0.0379,
wR2 = 0.0927
R1 = 0.0537,
wR2 = 0.0980
R1 = 0.0205, wR2 = 0.0503
R indices (all data)
R1 = 0.1129, wR2 = 0.2404
2.313 and À1.800
=
R1 = 0.0248, wR2 = 0.0520
Largest diff. peak and hole (e. ÅÀ3
)
0.302 and À0.233
0.361 and À0.444 0.636 and À0.280
(one Co–Npz and one Co–Nimine), Co–O bond, and a Co–Cl bond
(Fig. 4). The coordination geometry in this complex is also severely
distorted tetrahedral with bond angles (N/Cl–Co–N/O) varying
from 93.50(7)° to 124.04(7)°. The respective bond angles and bond
lengths in 5 and 6 are comparable (see figure captions).
predicted that ligands other than ethanol with similar hydrogen
bond donors may form similar cluster type compounds. Complex
7 is a prime example of this prediction where hydrogen bonding
between N–H protons of 3,5-Me2pza and chloride stabilizes the
cubane-like structure.
Complex 7, [{NiCl2(pza)}4], has a cubane-like Ni4Cl4 core with
each nickel center in an octahedral environment formed by two
nitrogen atoms from the pyrazolyl and amine groups, three
The nickel complex 9 crystallizes with two symmetry-indepen-
dent ionic pairs and one molecule of solvent water in an asymmet-
ric unit (Fig. 6). Each Ni(II) center in the cation resides in a
distorted octahedral environment formed by two pyrazolyl nitro-
gen atoms, three water oxygen atoms and a bromide. There are fif-
teen types of symmetry-independent hydrogen bonding
interactions present in this crystal. The nitrogen atoms are located
cis to each other in the basal plane formed by atoms N1–N3–O1–
O3. The Br atom is trans to the third aqua ligand. In both cations
the Ni–O bond lengths trans to the pyrazolyl nitrogen are
ꢀ0.08 Å longer than the other Ni–O distances. One possible expla-
nation is a displacement of electron density from the metal center
toward the pyrazolyl ring where a stronger bond is formed.
l
3-bridging chlorides and one terminal chloride (Fig. 5). This is
the first example of a pyrazolylamine tetranuclear cluster. The
Ni–Cl distances to the 3-bridging Cl atoms are substantially
l
longer than the terminal Ni–Cl bonds. The former can be separated
into two groups, the Ni–Cl(bridging) bonds that are parallel to the
Ni–Cl(terminal) bonds, and those perpendicular to them. The par-
allel Ni–Cl(l3-bridging) bonds average 2.469(12) Å, whereas the
perpendicular bonds (Ni1–Cl7, Ni1–Cl8, Ni3–Cl5, Ni4–Cl6) average
2.523(15) Å, and the difference is statistically significant. The
terminal Ni–Cl bonds are expectedly shorter at av. 2.367(11) Å,
because the bond distances usually shorten as the number of
atoms bridged diminishes. There are four relatively strong intra-
molecular hydrogen bonding interactions of the type N–HÁ Á ÁCl that
diagonally stitch the ligands on adjacent Ni atoms together.
Driessen and co-workers [22] have reported a similar tetranu-
clear nickel cubane-like structure [{NiCl(L)(EtOH)4}] (L = 3,5-dim-
ethylpyrazolylethanol). They suggested that intra-molecular
hydrogen bonding contributed to the stability of the cluster and
3.3. Ethylene oligomerization reactions
Complexes 1–9 were screened for their catalytic activities for
ethylene oligomerisation, using EtAlCl2 as co-catalyst. Only 7, 8
and 9 were found to be active for the oligomerisation of ethylene
to mainly 1-butene and 1-hexene. Of the three, complex 9 was
the most active. Complex 9 was also very selective towards the

M.K. Ainooson et al. / Polyhedron 53 (2013) 295–303
301
Fig. 2. Molecular structure of 1 shown with 50% thermal probability ellipsoids [26].
Only one symmetry independent cation is shown; the other cation, counterions,
solvents (see Fig. S1 for the full picture), and all H atoms are omitted. Selected bond
lengths [Å] and bond angles [o]: Fe(1)–O(1), 1.886(5); Fe(1)–O(2), 1.885(4); Fe(1)–
N(1), 2.170(6); Fe(1)–N(3), 2.126(5); Fe(1)–N(4), 2.226(6); Fe(1)–N(6), 2.115(5);
O(1)–C(8), 1.307(8). O(2)–Fe(1)–O(1), 95.2(2); O(1)–Fe(1)–N(3), 85.9(2); O(1)–
Fe(1)–N(6), 93.4(2); O(2)–Fe(1)–N(3), 98.9(2); O(2)–Fe(1)–N(6), 87.13(19); N(4)–
Fe(1)–N(3), 89.6(2); N(4)–Fe(1)–N(6); 84.5(2).
Fig. 5. Molecular structure of 7 depicted with 50% thermal probability ellipsoids
(see Fig. S2 for H-bonding interaction in the structure).26All H atoms are omitted for
clarity. Selected bond lengths [Å] and bond angles [°]: Ni(1)–N(3), 2.062(3); Ni(2)–
N(4), 2.069(3); Ni(2)–N(6), 2.070(3); Ni(3)–N(7), 2.048(3); Ni(3)–N(9), 2.052(3);
Ni(4)–N(12), 2.057(3); Ni(2)–Cl(2), 2.3808(10); Ni(3)–Cl(3), 2.3560(10). N(3)–
Ni(1)–N(1), 93.14(10); N(1)–Ni(1)–Cl(1), 93.72(8); N(3)–Ni(1)–Cl(1), 94.97(7);
N(1)–Ni(1)–Cl(5), 92.68(8); N(3)–Ni(1)–Cl(5), 89.14(7); Cl(1)–Ni(1)–Cl(5),
172.20(3); N(1)–Ni(1)–Cl(6), 164.10(7); N(3)–Ni(1)–Cl(6), 101.75(7); Cl(1)–Ni(1)–
Cl(6), 90.56(3).
Fig. 3. Molecular structure of 5 depicted with 50% thermal probability ellipsoids
[26]. Only one symmetry independent molecule without
H atoms is shown.
Selected bond lengths [Å] and bond angles [o]: Co(1)–O(1), 1.8876(17); Co(1)–N(1),
2.027(2); Co(1)–N(3), 1.970(2); Co(1)–Cl(1), 2.2458(9); O(1)–C(14), 1.314(3); N(3)–
C(8), 1.296(3). O(1)–Co(1)–N(1), 127.65(8); O(1)–Co(1)–N(3), 94.97(8); N(3)–
Co(1)–N(1), 94.02(8); N(3)–Co(1)–Cl(1), 119.21(7); N(1)–Co(1)–Cl(1); 109.49(6).
Fig. 6. Molecular structure of 9 shown with 50% thermal probability ellipsoids [26].
Only the Ni(1) cation is shown; the other cation, the BrÀ anions and the solvent
water molecule are omitted for clarity (see Fig. S3 for the full picture). Selected
bond lengths [Å] and bond angles [°]: Ni(1)–O(1), 2.0439(13); Ni(1)–O(2),
2.0773(13); Ni(1)–O(3), 2.1510(14); Ni(1)–N(1), 2.0705(15); Ni(1)–N(3),
2.0610(15); Br(1)–Ni(1), 2.5859(3). O(1)–Ni(1)–O(2), 88.92(6); O(1)–Ni(1)–N(1),
93.71(6); O(1)–Ni(1)–N(3), 173.71(6); N(1)–Ni(1)–O(2), 94.35(6); N(3)–Ni(1)–O(2),
91.79(6); N(3)–Ni(1)–N(1), 92.46(6).
dimerization and trimerization of ethylene. In most experiments a
greater percentage (>90%) of the oligomers produced from 9 were
1-butenes and 1-hexenes. From the large fractions of butenes and
hexenes in the product, it can be inferred that b-hydride elimina-
tion is favoured over ethylene insertion with this catalyst system.
No polymers were detected in any of these reactions. The Sun
group has reported several examples of N^N and N^N^N nickel cat-
alysts that catalyse the oligomerisation of ethylene [23]. Typically
the activities of phenanthroline nickel complexes are in the range
1.0 Â 104–5.8 Â 106 g molÀ1(Ni)hÀ1 [23]d,e. Other N^N nickel
Fig. 4. Molecular structure of 6 shown with 50% thermal probability ellipsoids [26].
The solvent molecules of acetone and all H are atoms omitted for clarity. Selected
bond lengths [Å] and bond angles [°]: Co(1)–O(1), 1.8899(16); Co(1)–N(1),
2.0404(19); Co(1)–N(3), 1.9743(19); Co(1)–Cl(1), 2.2293(7); O(1)–C(20), 1.321(3);
N(3)–C(18), 1.293(3). O(1)–Co(1)–N(1), 124.04(7); 124.04(7), O(1)–Co(1)–N(3),
95.41(7); O(1)–Co(1)–Cl(1), 115.06(5); N(3)–Co(1)–N(1), 93.50(7); N(1)–Co(1)–
Cl(1), 107.69(6); N(3)–Co(1)–Cl(1), 119.32(6).

302
M.K. Ainooson et al. / Polyhedron 53 (2013) 295–303
Table 2
Ethylene oligomerization data using catalysts 7, 8 and 9.
Entry
Catalyst
Time (min)
Temperature (°C)
Pressure (atm)
Yield (g)
Activity kg (mol Ni h)À1
Oligomer distribution
C4
C6
C8–C20
1
2
3
4
5
6
7
8
7
8
9
9
9
9
9
9
9
9
9
9
9
45
60
15
30
45
60
45
45
45
45
45
45
45
30
30
30
30
30
30
30
30
30
30
0
10
20
10
10
10
10
1
10.2
40.6
50.6
1 449
4 031
7 747
5 363
7 548
5 810
556
1 590
9 534
10 726
3 774
5 164
4 370
54
35
68
66
57
49
76
64
52
54
59
61
48
46
17
30
34
43
49
22
32
43
42
39
35
51
48
2
70.0
147.8
151.6
10.9
2
2
4
5
4
2
4
1
5
31.1
9
20
27
10
10
10
186.7
210.0
73.9
101.1
85.7
10
11
12
13
20
40
Conditions: 70 mL of toluene, 26
lmol catalyst loadings, Al: Ni = 540
catalysts that oligomerise ethylene include catalysts that contain
pyrazole [24]f and imine donor ligands [25] have typical activities
of 1.45 Â 106 g molÀ1(Ni)hÀ1 and 5.7 Â 105 g molÀ1(Ni)hÀ1 respec-
tively. Although complex 9 has a far superior activity (10.7 Â 106 -
g molÀ1(Ni)hÀ1) it is less selective towards the production of
1-butene compared to the phenanthroline [23]d,e and imine [25]
supported nickel catalysts. It is, however, not clear to us why com-
plex 8 forms a much larger amount of C8–C20 compared to com-
plexes 7 and 9.
ephenol ligand. With iron non-substituted pyrazolylimine ligand
bis(O^N^N) Fe(II) complexes were formed, while substituted pyra-
zolylimine ligands formed (O^N^N) Fe(II) and Co(II) complexes.
However, all the pyrazolylimine ligands were hydrolysed by Ni(II)
salts, forming pyrazolylamine nickel complexes. The pyrazolyl-
amine nickel complexes are unique in the oligomerising of ethyl-
ene to butenes and hexenes, without the butenes and hexenes
alkylating the toluene used as solvent with ethylaluminium dichlo-
ride as co-catalyst contrary to our recent findings.
In view of the high activity of 9 investigations of effect of time,
pressure and temperature on catalytic activity were performed
with this catalyst. The results from all the catalysis experiments
are shown in Table 2. The various reaction parameters affect cata-
lytic activity but no regular trend on the distribution of oligomer
products was observed. Ethylene oligomerization carried out at
different pressures resulted in increased activity as expected (Ta-
ble 2, entries 6–10). Activities were greater with increase in pres-
sure because more monomer was available to the catalytically
active sites. Catalyst lifetime of 9 was also investigated, with reac-
tions conducted for 15, 30, 45 and 60 min; with results that signify
the catalyst retains its activity for at least 45 min, after which
activity levels off. For example at a reaction time of 30 min, the
yield obtained was 50.6 g (7746.7 kg (mol Ni h)À1). Increase in
reaction time from 30 to 60 min resulted in decreased catalyst
activity (5810.0 kg (mol Ni h)À1). This was accompanied by in-
crease in viscosity of the reaction mixture. The decrease in cata-
lysts activity is likely due to decrease in the solubility of the
monomer; thus making less monomer available to the active site.
Increase in temperature also affected catalyst activity. At 0 °C
activity was 3374 kg (mol Ni h)À1, which is almost half that ob-
tained at 30 °C (7747 kg (mol Ni h)À1). In addition reactions were
highly exothermic, with the temperature of the ice bath used in
experiments rising to 26 °C unless there was constant addition of
ice to the cooling bath. At 40 °C catalyst activity decreased to
4370 kg (mol Ni h)À1, most likely due to catalyst decomposition
and possibly lower solubility of ethylene at elevated temperatures.
Interestingly the GC data for all the products from the catalysis
showed little or no evidence of Friedel–Crafts alkylation of toluene
by the ethylene oligomers formed in these reactions; an observa-
tion that is different from recent results reported by us [23].
Acknowledgements
Financial support for this work by the University of Johannes-
burg is gratefully acknowledged.
Appendix A. Supplementary data
CCDC 892497-892501 contains the supplementary crystallo-
graphic data for compounds 1, 5, 6, 7 and 9. These data can be ob-
tained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
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