Synthesis of titanium(IV) complexes containing 2,6-dimethylaniline substituted amino alcohols and their utilization in ethylene polymerizations

Article abstract of DOI:10.1016/S0022-328X(00)00907-4

Two new Ti(IV) complexes of the type [(OCHRCH₂N-2,6-Me₂Ph)Ti(NMe₂)₂] ₂ (where R=H, or ^tbutyl) have been synthesized by protonolysis of Ti(NMe₂)₄ with the corresponding N-substituted amino alcohol ligand. The two complexes were studied as Ziegler-Natta type polymerization catalysts in the presence of an excess of methylaluminoxane (MAO). It was found that ethylene polymerization activity for both catalysts increased by increasing the polymerization temperature from 25 to 70°C. If the catalysts were pretreated with trimethylaluminum, the activities increased at 25°C, but slightly decreased at 70°C. Both complexes were found to be ineffective for the polymerization of propylene. An X-ray crystal structure of [(OCH₂CH₂N-2,6-Me₂Ph)Ti(NMe₂) ₂]₂ shows the complex to be dimeric in the solid state, with bridging through the amino alcohol oxygen atoms. This complex crystallizes in the monoclinic system space group, P2₁/n with a=9.181(6), b=13.766(3), c=13.002(3), β=93.08(3) and Z=4.

Full text of DOI:10.1016/S0022-328X(00)00907-4

Journal of Organometallic Chemistry 625 (2001) 95–100
www.elsevier.nl/locate/jorganchem
Synthesis of titanium(IV) complexes containing 2,6-dimethylaniline
substituted amino alcohols and their utilization in ethylene
polymerizations
Barrie Rhodes, James C.W. Chien, John S. Wood, A. Chandrasekaran,
Marvin D. Rausch *
Department of Chemistry, Uni6ersity of Massachusetts, Amherst, MA 01003, USA
Received 22 August 2000; received in revised form 13 November 2000; accepted 13 November 2000
Abstract
t
Two new Ti(IV) complexes of the type [(OCHRCH2N-2,6-Me2Ph)Ti(NMe2)2]2 (where R=H, or butyl) have been synthesized
by protonolysis of Ti(NMe2)4 with the corresponding N-substituted amino alcohol ligand. The two complexes were studied as
Ziegler–Natta type polymerization catalysts in the presence of an excess of methylaluminoxane (MAO). It was found that
ethylene polymerization activity for both catalysts increased by increasing the polymerization temperature from 25 to 70°C. If the
catalysts were pretreated with trimethylaluminum, the activities increased at 25°C, but slightly decreased at 70°C. Both complexes
were found to be ineffective for the polymerization of propylene. An X-ray crystal structure of [(OCH2CH2N-2,6-
Me2Ph)Ti(NMe2)2]2 shows the complex to be dimeric in the solid state, with bridging through the amino alcohol oxygen atoms.
This complex crystallizes in the monoclinic system space group, P21/n with a=9.181(6), b=13.766(3), c=13.002(3), i=93.08(3)
and Z=4. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Polymerization; Titanium; Non-cyclopentadenyl; Ethylene
pared two titanium(IV) complexes containing chelating
ligands with both an h1-amido and an h1-alkoxy link-
age with the general formula [(OCHRCH2N-2,6-
1. Introduction
The organometallic chemistry of titanium has been
dominated by complexes containing cyclopentadienyl
ligands [1]. This is reflected in the fact that most
homogeneous Ziegler–Natta catalyst precursors are
also based on the cyclopentadienyl framework [2a].
More recently, however, non-cyclopentadienyl based
catalyst precursors have become more prominent in the
literature [2b]. Some examples are titanium and zirco-
nium complexes containing chelating dialkoxide ligands
[3,4] and chelating diamido ligands [5–19]. These cata-
lyst precursors have been used for the Ziegler–Natta
polymerization of olefins. In some instances, living
polymerization was observed [15].
t
Me2Ph)Ti(NMe2)2]2 (where R=H or butyl) and have
studied their ability to polymerize a-olefins in the pres-
ence of methylaluminoxane (MAO). This ligand design
was chosen to discover how the effects of using a
ligand, containing both a covalently bonded oxygen
and nitrogen group would affect the catalytic properties
of the complex. Previous research has concentrated
mainly on symmetric ligands containing either two
nitrogen or oxygen containing ligands.
2. Results and discussion
As part of our program to synthesize novel Ziegler–
Natta polymerization catalyst precursors, we have pre-
2.1. Synthesis
The amino alcohol ligands were easily prepared in
good yield by nucleophilic ring opening of epoxides.
Reaction of the lithium salt of 2,6-dimethylaniline (1)
* Corresponding author. Tel.: +1-413-5452291; fax: +1-413-
5454490.
E-mail address: rausch@chem.unmass.edu (M.D. Rausch).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00907-4

96
B. Rhodes et al. / Journal of Organometallic Chemistry 625 (2001) 95–100
for this dimethylamido group (l 2.39 and 3.10),
whereas in the high temperature-limiting spectrum
(60°C), the dimethylamido group appears as a sharp
singlet (l 2.73). We interpret this to be a consequence
of restricted rotation about one of the Ti–N(amido)
groups in 4b. At all temperatures studied, the other
dimethylamido group appeared as a sharp singlet (ca. l
3.38). Additionally, the methyl substituents on the aro-
matic group are also non-equivalent, appearing as two
singlets at all temperatures studied (ca. l2.33 and 2.59).
This may be a consequence of restricted rotation about
the N–Cipso bond1.
2.2. Polymerization studies
Complexes 4a and 4b were examined as potential
Ziegler–Natta polymerization catalysts in the presence
of MAO. Polymerizations of ethylene were carried out
at two different temperatures, either with or without
preactivation with trimethylaluminum (TMA). It has
previously been reported that the preactivation of zirco-
nium bis(dimethylamido) compounds with TMA
methylates the complexes, and thus the complex is more
easily activated with MAO [21]. The results are summa-
rized in Table 1. Attempts to polymerize propylene
under identical conditions resulted in no polymer being
formed. Perhaps most notable is the effect of tempera-
ture on the polymerizations in the absence of TMA. In
general, polymerization at room temperature afforded
only a small amount of polymer, with activities in the
low 104–105 range. Increasing the temperature to 70°C
Scheme 1.
with ethylene oxide (2a) or 3,3-dimethyl-1,2-epoxybu-
tane (2b) in the presence of tetramethylethylenediamine
(TMEDA), followed by hydrolysis, formed the amino
alcohol ligands (Scheme 1). We were also able to pre-
pare the unsubstituted amino alcohol (3a) by modifica-
tion of
a literature procedure [20]. Reaction of
2-chloroethanol with an excess of 2,6-dimethylaniline,
followed by the addition of a base, formed the amino
alcohol in good yield (Scheme 2).
The titanium(IV) complexes were synthesized via the
aminolysis of tetrakis(dimethylamido) titanium in
refluxing hexane (Scheme 3). The products formed were
isolated as air-sensitive yellow or orange solids. The
unsubstituted complex 4a was isolated as crystalline
produced
a
significantly greater amount of
polyethylene. We attribute this to the fact that these
compounds are difficult to activate at ambient tempera-
tures, i.e. alkylation of the titanium species is slow. This
process is much more spontaneous at the higher tem-
perature of 70°C. In the case where the complexes were
preactivated with TMA, an increase in activity was
observed at room temperature. The activities at 70°C
are slightly lower when preactivated with TMA, indi-
cating that the methylated titanium complexes are
slightly less stable at these temperatures. In all cases,
t
orange needles, whereas for the butyl substituted com-
plex 4b, it was not possible to grow crystals suitable for
X-ray work due to the high solubility of 4b in hydrocar-
bon solvents. It formed only a yellow, microcrystalline
powder.
The proton NMR spectrum of 4b was found to be
temperature dependent. At all temperatures studied
(−80 to +60°C), two dimethylamido groups were
found to be magnetically non-equivalent. At ambient
temperatures, one of the dimethylamido groups appears
as a broad, featureless peak (l 2.78). The low tempera-
ture-limiting spectrum (−20°C) exhibits two singlets
1 Restricted rotation was also observed in several complexes con-
taining a 2,6-diisopropylphenyl group [8,10,15–17,19].
Scheme 2.

B. Rhodes et al. / Journal of Organometallic Chemistry 625 (2001) 95–100
97
Scheme 3.
Table 1
Ethylene polymerization with catalyst precursors 4a, 4b/MAO
a
Catalyst
Preactivated with
TMA b
Polymerization temperature
(°C)
Polymer yield
(g)
Activity c
m.p. of polymer
(°C)
Mw of polymer d
(g mol−1
)
4a
4a
4b
4b
4a
4a
4b
4b
No
No
No
No
Yes
Yes
Yes
Yes
25
70
25
70
25
70
25
70
0.011
0.59
0.102
0.30
0.18
0.50
0.12
0.10
1.2×104
1.1×106
1.1×105
5.8×105
1.9×106
9.6×105
1.3×105
1.9×105
134.1
126.2
134.0
128.9
135.9
127.9
131.7
132.6
6.8×105
1.2×105
2.5×105
1.1×105
7.3×105
4.2×105
1.8×105
1.1×105
a Polymerization conditions; [Ti]=50 mM; [Al]:[Zr]=4000:1; monomer pressure=15 psi; time of polymerization=1 h.
b Catalyst preactivated with five equivalents of TMA for 20 min before polymerization initiation.
c Activity expressed in units of g polymer/(mol Ti·[C2H4] h).
d Mw determined by viscometry in decalin at 135°C.
the activities shown here are similar to the activities
exhibited by the chelating alkoxide complexes described
by Schaverien et al. which polymerize ethylene with
activities in the range 105–106 gPE/mol cat h [3].
plex 5 [25], which is the closest structurally to that
described here, the principal difference being the phenyl
substituents on the chelate ring of the ligand. The bond
2.3. Crystal and molecular structure of
(OCH2CH2N-2,6-Me2Ph)Ti(NMe2)2 (4a)
Fig. 1 gives an ORTEP [22] plot of the complex 4a,
together with the atom labelling for the asymmetric
unit. Table 2 shows the crystal data and structure
refinement for 4a and Table 3 lists selected bond dis-
tances and angles for the metal coordination
environment.
The complex is a centrosymmetric dimer (the center
of inversion being crystallographically imposed) with
bridging oxygen atoms, and a distorted trigonal bipyra-
midal geometry around each titanium atom. The obser-
vation of bridging oxygen atoms to form dimeric and
higher polymeric species is a common feature of tita-
nium chemistry [23–25], and similar dimeric geometries
have been reported for related complexes containing
N-substituted amino-alcohols [25]. Fig. 2 shows com-
Fig. 1. Molecular structure of 4a.

98
B. Rhodes et al. / Journal of Organometallic Chemistry 625 (2001) 95–100
Table 2
ing the lower steric restrictions due to the chelate ring.
There is no significant difference between the equatorial
and axial Ti–NMe2 distances, and as for 5 [25], these
are a little shorter than the Ti–N(Ar) distance.
Unfortunately, we have been unable to grow ade-
quate crystals of 4b for X-ray analysis as 4b is difficult
to crystallize even when in a very pure state.
Crystal data and structure refinement for 4a
Empirical formula
Formula weight
Temperature (K)
C14H25N3OTi
299.27
293(2)
,
Wavelength (A)
0.71073, monochromated
Mo Ka
Monoclinic
P21/n
Crystal system
Space group
,
a (A)
9.181(6)
13.766(3)
13.002(3)
93.08(3)
4
1.211
5.190
2009
,
b (A)
3. Summary and conclusions
,
c (A)
i (°)
We have prepared successfully two new titanium (IV)
complexes based on ligands containing 2,6-dimethylani-
line substituted amino alcohols. These catalysts have
been shown to polymerize ethylene in the presence of
an excess of MAO, exhibiting greater activities at 70°C
than room temperature. We propose that methylation
of the catalyst precursors is difficult at room tempera-
ture by MAO, or the TMA in it, but is spontaneous at
70°C. If the catalysts were preactivated (methylated)
with trimethylaluminum (TMA) prior to the initiation
of the polymerization, the activities were increased at
room temperature compared to when the catalysts were
not treated with TMA. The activities at higher temper-
atures were slightly decreased, probably due to the
instability of the methylated titanium precursor at this
temperature.
Z
Dcalc (g cm−3
)
Absorption coefficient (cm−1
Total independent reflections
measured
)
Final R indices [I\2|(I)] a
Goodness-of-fit
R1=0.0540, wR2=0.0996
S=1.12
a R1=S ꢀꢀ Fo ꢀ−ꢀ Fc ꢀꢀ /S ꢀ Foꢀ. wR2=[S(F2o−Fc2)2]/S[w(Fo2)2]1/2
.
Table 3
a
,
Selected bond lengths (A) and bond angles (°) for 4a
Bond lengths
Ti–N(1)
Ti–N(3)
Ti–O%
1.938(5)
1.891(5)
1.952(4)
1.419(7)
1.478(8)
1.435(9)
1.465(9)
Ti–N(2)
Ti–O
Ti–Ti%
C(1)–C(2)
N(1)–C(7)
N(2)–C(4)
N(3)–C(6)
1.882(6)
2.100(4)
3.270(2)
1.497(9)
1.429(8)
1.466(8)
1.454(8)
O–C(1)
C(2)–N(1)
N(2)–C(3)
N(3)–C(5)
4. Experimental
Bond angles
N(2)–Ti–N(3)
N(3)–Ti–N(1)
N(3)–Ti–O%
N(2)–Ti–O
101.3(3)
N(2)–Ti–N(1)
N(2)–Ti–O%
N(1)–Ti–O%
N(3)–Ti–O
O%–Ti–O
112.2(2)
4.1. General procedures
99.1(2)
94.8(2)
98.1(2)
78.0(2)
114.3(2)
127.4(2)
160.0(2)
72.5(2)
All experiments were performed under a dry argon
atmosphere using standard Schlenk techniques. The
argon was purified by deoxygenating with BTS catalyst
and drying with molecular sieves and P2O5. Tetrahy-
drofuran (THF) was predried over sodium wire, dis-
tilled from sodium under argon, and finally distilled
from Na/K alloy under argon. Diethyl ether was
predried over sodium wire and distilled from Na/K
alloy under argon. Toluene, hexane, and pentane were
distilled from Na/K alloy under argon. Methylene chlo-
ride was distilled from calcium hydride. Deuterated
solvents were stored over activated molecular sieves
under an argon atmosphere.
N(1)–Ti–O
a Symmetry transformations used to generate equivalent atoms:
−x, −y, −z+2.
Methylaluminoxane was purchased from Akzo.
Butyllithium (1.6 M in hexanes), 2-chloroethanol and
ethylene oxide were purchased from Aldrich and used
without further purification. 2,6-Dimethylaniline was
purchased from Aldrich and distilled from CaH2, in
vacuo, prior to use. 3,3-Dimethyl-1,2-epoxybutane was
purchased from Lancaster and used without further
Fig. 2. Molecular structure of 5.
distances and angles involving the titanium atom in 4a
are closely similar to those reported for 5 [25], the only
change of any significance being the slightly larger
difference in the Ti–O bond lengths, the equatorial
purification.
N,N,N%,N%-tetramethylethylenediamine
,
oxygen (of the trigonal bipyramid) being 0.15 A closer
to the metal than the axial oxygen, and perhaps reflect-
(TMEDA) was purchased from Aldrich and distilled
from Na prior to use. Tetrakis(dimethylamido) tita-

B. Rhodes et al. / Journal of Organometallic Chemistry 625 (2001) 95–100
99
nium was prepared by the literature method [26]. Celite
was purchased from Fischer Scientific and used without
pretreatment.
The resulting solid was dried in vacuo and the product
crystallized from pentane to afford the title compound
as a pale yellow crystalline solid (9.98 g, 72% yield);
m.p. 71–72°C. Anal. Calc. for C14H23NO: C, 75.97; H,
10.47; N, 6.33. Found: C, 76.15; H, 10.58; N, 6.35%.
1H-NMR spectra were recorded on a Varian XL-200
spectrometer with tetramethylsilane (Me4Si) as an inter-
nal standard at ambient temperatures unless otherwise
stated. Melting points of the polymers were obtained
using a Perkin–Elmer model DSC-4 differential scan-
ning calorimeter. Microanalyses were performed by the
Microanalytical Laboratory, University of Massachu-
setts, Amherst, MA.
t
1H-NMR (CDCl3) l 0.93 (s, 9H, Bu), 2.31 (s, 6H,
Ar–CH3), 2.75 (bs, 1H, OH), 2.73–2.84 (t, 1H, –CH),
3.11–3.19 (dd, 1H, –CH2), 3.31 (bs, 1H, NH), 3.39–
3.46 (dd, 1H, –CH2), 6.81–7.03 (m, 3H, aromatic-H).
4.5. Preparation of [(2,6-Me2PhNC2H4O)Ti(NMe2)2]2
(4a)
4.2. Preparation of 2,6-Me2PhNHLi (1)
Ligand 3a (2.66 g, 16.1 mmol) was added in portions
to a solution of Ti(NMe2)4 (3.60 g, 16.1 mmol) in
hexane (50 ml) at 0°C. The mixture was placed under
reflux overnight and the solution cooled to r.t. The
mixture was filtered and the resulting orange solid dried
in vacuo. The product was crystalized from toluene at
−20°C as orange needles (3.76 g, 78.0% yield). Anal.
Calc. for C14H25N3OTi: C, 56.19; H, 8.42; N, 14.04.
Butyllithium (40.0 ml, 64 mmol) was slowly added to
a solution of 2,6-dimethylaniline (7.75 g, 64 mmol) in
hexane (75 ml) at 0°C. A white solid formed immedi-
ately. The mixture was warmed to room temperature
(r.t.) and stirred for 2 h. The hexane was removed by
cannula filtration and the resulting white solid dried in
vacuo yielding a white, pyrophoric powder (8.03 g, 98%
yield).
1
Found: C, 56.98; H, 8.50; N, 12.87%. H-NMR (C6D6)
l 3.01 (s, 6H, Ar–CH3), 3.46 (t, 2H, CH2), 4.60 (t, 2H,
–CH2), 6.92–7.18 (m, 3H, aromatic-H).
4.3. Preparation of 2,6-Me2PhN(H)C2H4OH (3a)
Method (a): ethylene oxide (1.76 g, 40 mmol) was
added to a solution of 1 (5.0 g, 39.3 mmol) in THF (50
ml) and the mixture was stirred at room temperature
for 2 h. The mixture was hydrolyzed with water (50 ml)
and the product extracted with diethyl ether (3×50
ml). The organic layers were combined, dried (MgSO4)
and the solvents removed. The resulting oil was distilled
at 66–68°C (0.01 mmHg) to yield a colorless oil (2.77 g,
4.6. Preparation of
[(2,6-Me2PhNC2H3(tBu)O)Ti(NMe2)2]2 (4b)
Ligand 3b (2.00 g, 9.04 mmol) was added in portions
to a solution of Ti(NMe2)4 (2.03 g, 9.04 mmol) in
hexane (50 ml) at 0°C. The mixture was placed under
reflux overnight and the reaction mixture was then
cooled to r.t. The solution was filtered from any insolu-
ble impurities and cooled to −78°C. The product was
isolated as a highly hydrocarbon soluble yellow powder
(1.76 g, 54.8% yield). Anal. Calc. for C18H33N3OTi: C,
60.84; H, 9.36; N, 11.82. Found: C, 60.95; H, 9.59; N,
1
43% yield). H-NMR (CDCl3) l 2.29 (s, 6h, Ar–CH3),
3.05 (bs, 2H, NH/OH), 3.09 (t, 2H, CH2), 3.73 (t, 2H,
CH2), 6.79–7.01 (m, 3H, aromatic-H). Method (b) [20]:
2,6-dimethylaniline (145 g, 1.2 mol) and 2-
chloroethanol (29.0 g, 0.36 mol) were stirred at 100°C
for 48 h. After the mixture was cooled to r.t., water
(100 ml) was added and solid KOH was added until the
mixture was basic to litmus. The organics were ex-
tracted in diethyl ether (3×100 ml). The organic layers
were combined, dried (MgSO4) and the residue distilled
at 66–68°C (0.01 mmHg) to yield a colorless oil (38.7 g,
1
t
11.61%. H-NMR (−20°C) (C7D8): l 0.94 (s, 9H, Bu),
2.33 (s, 3H, Ar–CH3), 2.39 (s, 3H, NMe2), 2.61 (s, 3H,
Ar–CH3), 3.05 (dd, 1H, –CH2), 3.10 (s, 3H, NMe2),
3.38 (s, 6H, NMe2), 4.18 (dd, 1H, –CH2), 4.37 (t, 1H,
–CH), 6.96–7.13 (m, 3H, aromatic-H). 1H-NMR
t
(20°C) (C7D8): l 0.94 (s, 9H, Bu), 2.33 (s, 3H, Ar–
CH3), 2.59 (s, 3H, Ar–CH3), 2.78 (bs, 6H, NMe2), 3.08
(dd, 1H, –CH2), 3.38 (s, 6H, NMe2), 4.16 (dd, 1H,
–CH2), 4.38 (t, 1H, –CH), 6.95–7.10 (m, 3H, aromatic-
H). 1H NMR (60°C) (C7D8): l 0.96 (s, 9H, tBu), 2.32 (s,
3H, Ar–CH3), 2.55 (s, 3H, Ar–CH3), 2.73 (s, 6H,
NMe2), 3.06 (dd, 1H, –CH2), 3.48 (s, 6H, NMe2), 4.15
(dd, 1H, –CH2), 4.39 (t, 1H, –CH), 6.81–7.10 (m, 3H,
aromatic-H).
1
65% yield). H-NMR data as above.
4.4. Preparation of 2,6-Me2PhN(H)C2H3(tBu)OH (3b)
3,3-Dimethyl-1,2-epoxybutane (6.3 g, 62.9 mmol) was
added to a solution of 1 (8.03 g, 62.9 mmol) and
N,N,N%,N%-tetramethylethylenediamine (TMEDA) (9.49
ml, 62.9 mmol) in THF (75 ml). The solution was
placed under reflux overnight. The mixture was hy-
drolyzed with water (75 ml) and the product extracted
with diethyl ether (3×60 ml). The organic layers were
combined, dried (MgSO4), and the solvents removed.
4.7. Polymerization procedure
A 250-ml glass pressure bottle was sealed under an
argon atmosphere. Freshly distilled toluene (50 ml) was

100
B. Rhodes et al. / Journal of Organometallic Chemistry 625 (2001) 95–100
added via a syringe, and pressurized with the appropri-
ate monomer (15 psi). The appropriate amount of
methylaluminoxane (MAO) was added and the bottle
was placed in a bath at the desired polymerization
temperature and stirred for ten min. The catalyst pre-
cursor (preactivated with five equivalents of TMA for
20 min when necessary) in toluene was then added, and
the mixture was stirred until the desired reaction time
was reached. The reaction mixture was subsequently
quenched with 2% HCl in methanol (200 ml), filtered,
and dried in a vacuum oven at 70°C.
References
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[2] (a) M. Bochmann, J. Chem. Soc. Dalton. Trans. 3 (1996) 255.
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G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. Int.
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4.8. Crystal structure determination
X-ray diffraction data for an orange needle-like crys-
tal of 4a were collected on an Enraf–Nonius CAD4
diffractometer at room temperature, using monochro-
mated Mo Ka radiation and the ꢀ/2q scan mode.
Empirical absorption corrections, based on psi scans
were made to the data. Details of unit cell dimensions
etc. are summarized in Table 2.
The structure was solved by direct methods using
SHELXS-86 [27] and refined by full-matrix least squares
on F2 using all 2009 independent reflections using the
SHELXL-93 program [28]. The final residual, R1 (based
on F) for the 1329 reflections with I]2|(I) was 0.054.
Hydrogen atoms based on a riding model with a C–H
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(1997) 1810.
,
distance of 1.05 A were included in the refinement.
Neutral atom scattering factors for non-hydrogen
atoms were taken from International Tables [29], and
anomalous dispersion corrections were included [30].
The hydrogen atom scattering factor used, is that tabu-
lated by Stewart et al. [31].
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(1997) 4415.
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10008.
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(1996) 5586.
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Gibson, B.S. Kimberley, A.J.P. White, D.J. Williams, P.
Howard, Chem. Commun. 3 (1998) 313.
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[28] G.M. Sheldrick, SHELXL-93, Program for crystal structure refine-
ment, Germany.
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 152128 for compound 4a.
Copies of this information may be obtained from The
Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK (fax: +44-1233-336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.
cam.ac.uk).
[29] International Tables for X-ray Crystallography, vol. IV, Kynoch
Press, Birmingham, UK, 1974, pp. 99–149.
Acknowledgements
[30] D.T. Cromer, D.J. Lieberman, J. Chem. Phys. 53 (1970) 1891.
[31] R.F. Stewart, R.F. Davidson, W.J. Simpson, J. Chem. Phys. 42
(1965) 3175.
We wish to thank Dr L.C. Dickinson for assistance
in acquiring V.T. NMR data.
.