Synthesis and characterisation of a novel mixed donor P,O,P' nixantphos ligand and its metal complex

Article abstract of DOI:10.1016/j.molstruc.2015.10.083

The complex [(NixC8OH)Ir(cod)Cl] 4 has been synthesized and structurally characterized by NMR, IR and single crystal X-ray diffraction. The synthesis and characterisation of the novel ligand NixC8OH is also presented. The coordination around Ir is trigonal bipyramidal with both P groups of the NixC8OH ligand bound in a bis-equatorial mode. The bis-chelating cod (C₈H₁₂) ligand occupies the remaining equatorial position and an axial position. This mode of bonding has resulted in a large bite angle (P1-Ir-P2) of 102.92(12)° for the title complex 4. The IR and NMR data further support the elucidated structure. Thermal analyses of 4 indicate that it is thermally stable up to a decomposition temperature of >400 °C.

Full text of DOI:10.1016/j.molstruc.2015.10.083

Journal of Molecular Structure 1106 (2016) 5e9
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis and characterisation of a novel mixed donor P,O,P'
nixantphos ligand and its metal complex
Thashree Marimuthu, Muhammad D. Bala*, Holger B. Friedrich**
School of Chemistry & Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The complex [(NixC8OH)Ir(cod)Cl] 4 has been synthesized and structurally characterized by NMR, IR and
single crystal X-ray diffraction. The synthesis and characterisation of the novel ligand NixC8OH is also
presented. The coordination around Ir is trigonal bipyramidal with both P groups of the NixC8OH ligand
bound in a bis-equatorial mode. The bis-chelating cod (C8H12) ligand occupies the remaining equatorial
position and an axial position. This mode of bonding has resulted in a large bite angle (P1eIreP2) of
102.92(12)ꢀ for the title complex 4. The IR and NMR data further support the elucidated structure.
Thermal analyses of 4 indicate that it is thermally stable up to a decomposition temperature of >400 ꢀC.
© 2015 Elsevier B.V. All rights reserved.
Received 4 August 2015
Received in revised form
23 October 2015
Accepted 26 October 2015
Available online 30 October 2015
Keywords:
Nixantphos
Iridium complex
Bite angle
Catalyst
1. Introduction
and some fully characterised cationic and neutral complexes
exhibiting merꢁk3ꢁP,O,P coordination modes via the oxygen atom
The ability to control the shape of a coordination compound in
catalysis has been an area of intense research interest. The pio-
neering work of Trofimenko in the mid 1960's introduced tris(-
pyrazol-1-yl)borate anions (Tp) and their analogues into the field of
coordination chemistry [1e4]. These ligands were characterised by
their unique fac binding (Fig.1a) to a metal centre and are known as
scorpionate ligands. Other tridentate ligands including the pincer
types preferentially coordinate to metals in a mer binding mode
(Fig. 1b). Additionally, PCP pincer ligands due to the disposition of
their donor atoms give complexes with Ir that can activate highly
inert CeH bonds [5]. Based on the success of tridentate ligands we
became interested in the design and complexation of a new class of
xanthene based ligands.
of the xanthene ring have been reported (Fig. 1b) [15e22]. It is
important to highlight that recently Weller and co-workers have
reported the structure of the complex [Ir(k m-
3(Xantphos)(H)(
H)]2[BArF
]
4 2
which also displayed the rare fac-k3ꢁP,O,P coordina-
tion [15]. This study, therefore serves as another basis towards the
unambiguous preparation of fac coordinating k3 ligands derived
from nixantphos 1 where a third donor has been functionalised
onto the backbone.
2. Experimental
2.1. Materials and methods
Herein we report the synthesis of a mixed donor P,O,P' scorpi-
onate type ligand 2 and the structure of its iridium metal complex 4
(Scheme 1). The ligand backbone nixantphos 1 belongs to the
xanthene family introduced by van Leeuween and co-workers [7,8].
These ligands have been extensively reviewed for their P,P cis-k2
coordination mode to transitional metal centres [9e14] (Fig. 1c),
Chemicals used were of reagent grade and reactions were car-
ried out in distilled and dried solvents using standard Schlenk tube
techniques under inert and dry nitrogen atmosphere. Iridium tri-
chloride hydrate (IrCl3$xH2O) was obtained from Johnson Matthey
and used as received. The precursor (8-bromooctyloxy)(t-butyl)
dimethylsilane was prepared by literature method [23]. The
dimeric chloro bridged precursor di-m-bis(1,5-cycloocatadiene)
diiridium(I) [Ir(cod)Cl]2 2 was prepared by the reduction of IrCl3 in
the presence of excess 1,5-cycloctadiene (cod) (Fluka ꢂ98%) in
aqueous ethanol (Merck absolute ACS grade) [24]. FTIR spectra
were recorded in the 4000e400 cmꢁ1 region on a Perkin Elmer
attenuated total reflectance (ATR) infrared spectrophotometer. 1H,
* Corresponding author.
** Corresponding author.
E-mail
addresses:
bala@ukzn.ac.za
(M.D.
Bala),
friedric@ukzn.ac.za
(H.B. Friedrich).
http://dx.doi.org/10.1016/j.molstruc.2015.10.083
0022-2860/© 2015 Elsevier B.V. All rights reserved.

6
T. Marimuthu et al. / Journal of Molecular Structure 1106 (2016) 5e9
(4 ꢃ 10 mL). The collective fractions were dried over anhydrous
sodium sulphate and purified by column chromatography with 10%
hexane/ethyl acetate elution. The resulting oil was dissolved in THF
(25 mL), and tetra-N-butylammonium fluoride added and solution
was left to stir overnight at room temperature. Thereafter, similar
aqueous work-up was carried out. The crude product was purified
by column chromatography with 20% ethyl acetate/hexane elution
to afford 2 in 28% (38 mg oil). IR nmax (cmꢁ1): 3343(m),3053(m),
2924(m), 2853(m), 1726(m), 1462(s), 1435(s), 1412(s), 1276(m),
1222(m), 743(s), 693(s); HR-MS (ESI) (m/z):[MþH]þcalcd. for
C44H44NO2P2, 680.2842; found, 680.2843. Details of the NMR data
are presented in Table 1.
Synthesis of 4. Compound 3 (30 mg, 0.04 mmol) was added to
[Ir(cod)Cl]2 (13.4 mg, 0.02 mmol) in THF (6 mL) under an inert
argon atmosphere at room temperature. Immediate discoloration
was observed, and the mixture was allowed to stir overnight. The
THF solvent was removed in vacuo and the resulting precipitate was
washed with hexane (3 ꢃ 6 mL), and extracted with dichloro-
methane (2 ꢃ 6 mL). The dichloromethane was removed in vacuo
and the complex 4 dried under high vacuum overnight to afford 4 in
52% (21 mg yellow powder). Melting point 504 K, DSC 513 K; IR
nmax (cmꢁ1):1938(s), 2920(m), 1585(m), 1489(s); HRMS (ESI) (m/z):
[M]þ - Cl calcd for C52H55IrNO2P2, 980.3332; found, 980.3333; EA:
Calculated for C52H55ClIrNO2P2: C, 61.5; H, 5.5, N; 1.4. Found: C,
61.0; H, 5.2; N, 1.3. Details of NMR data are presented in Table 2.
Fig. 1. Comparison between (a) the fac coordination of a k
3 tripodal ligand (b) the mer
binding of a k3 pincer ligand and (c) cis-k2 coordination of a bidentate ligand [6].
2.1.2. Structure analysis and refinement
Single-crystal structure determination by X-ray diffraction was
performed on a Bruker APEXII CCD area-detector diffractometer
with graphite monochromated Mo Ka radiation (50 kV, 30 mA)
using the APEX 2 data collection software [25]. The collection
method involved
u
-scans of width 0.5ꢀ and 512 ꢃ 512 bit data
frames. Data reduction was carried out using the SAINTþ software
[26] and face indexed absorption corrections were made using the
software XPREP. The crystal structure was solved by direct methods
using SHELXTL. Non-hydrogen atoms were first refined isotropically
followed by anisotropic refinement by full matrix least-squares
calculations based on F2 using SHELXTL. Hydrogen atoms were
first located in the difference map then positioned geometrically
and allowed to ride on their respective parent atoms with
CeH ¼ 0.95 Å and Uiso(H) ¼ 1.2Ueq(C). Diagrams and publication
material were generated using SHELXTL [27], PLATON [28] and
ORTEP-3 [29]. The n-octanol tail was found to be disordered. As a
consequence it was refined over two positions with final occu-
pancies of 0.414(14) and 0.586(14) using SADI restraints. Further
details of the X-ray structural analysis are given in Table 3, while
selected bond lengths and angles for 4 are listed in Table 4. The
asymmetric unit was found to contain 2 molecules and 1 fragment
of 2-propanol.
Scheme 1. Synthesis of the title complex 4 from ligand 2.
31P and 13C NMR measurements were collected at 298 K with
Bruker AVANCE III 400 MHz and 600 MHz spectrometers using
5 mm tubes and deuterated chloroform as the solvent. Coupling
constants (J) are given in Hz. The melting point was determined
using a Gallenkamp melting point apparatus and is uncorrected.
High resolution mass spectroscopy was obtained with the Bruker
micrOTOF-Q II instrument operating at ambient temperatures,
under electron spray ionisation conditions (ESI), using a sample
concentration of approximately 1 ppm. Differential scanning calo-
rimetry (DSC) measurements (Al2O3 reference standard) were
performed on a TA Thermo Gravimetric Analyser at a heating rate of
10 ꢀC/min under nitrogen atmosphere. Elemental analyses were
performed on a LECO CHNS-932 elemental analyser.
3. Results and discussion
3.1. Synthesis
Ligand 2 was prepared by the sodium hydride induced alkyl-
ation of nixantphos 1 with a protected tail precursor. Deprotection
by standard methods of the resulting product resulted in the O-
donor ligand 2. Complex 4 was readily prepared from [Ir(cod)Cl]2
metal precursor and ligand 2 at room temperature to afford 52%
yield of a yellow powder. Single crystals of 4 suitable for X-ray
analyses were grown by slow diffusion of 2-propanol into a
dichloromethane solution of the complex.
2.1.1. Synthesis of compounds 2 and 4
Synthesis of 2. Nixantphos 1 (200 mg, 0.36 mmols) was dis-
solved in DMF (4 mL). To the orange reaction mixture, NaH (400 mg,
0.72 mmol) was added. (8-bromooctyloxy)(t-butyl)dimethylsilane
(180 mg, 0.63 mmol) was slowly added, and the mixture stirred at
100 ꢀC overnight. The reaction was worked up by the addition of
water (10 mL) and the organic phase extracted with ethyl acetate

T. Marimuthu et al. / Journal of Molecular Structure 1106 (2016) 5e9
7
Table 1
NMR spectroscopic data for 2.
aCarbon number
1H (
d
¼ ppm)
13C (
d
¼ ppm)
12, 19
11, 18
10, 17
6.4 bd [J (H,H) ¼ 7.8]
6.6 t [J (H,H) ¼ 7.8]
6.0 m
111.7
123.5
125.1
C46eC51
C46
C52
1.6e1.2(12H)
32.7, 29.4, 29.4, 26.9,25.7, 24.7
63.0
44.7
128.1 t,[J(P,C) ¼ 3.4 Hz],
133.9 d, [J(P,C) ¼ 10.3 Hz], 128.2 m
147.1 m
137.0 t, [J(P,C) ¼ 6.5 Hz]
133.2 t, [J(P,C) ¼ 1.8 Hz]
124.8 d, [J(P,C) ¼ 6.3 Hz]
3.6 [t, J ¼ 6.5 Hz]
3.5 [t, J ¼ 7.8 Hz]
7.5e7.1 m (20H)
Phenyl groups on P1 & P2
C14,15
C21,31,41,51
C13,20
e
e
e
e
C9,16
a
Carbon numbers relative to the numbering scheme for 2 in Scheme 1.
Table 2
NMR spectroscopic data for 4.
aCarbon number
1H (
d
¼ ppm)
13C (
d
¼ ppm)
12, 19
11, 18
10, 17
6.6 bd [J (H,H) ¼ 7.8]
6.8 t [J (H,H) ¼ 7.9]
6.2 m (2H)
112.6
123.5 t, [J(P,C) ¼ 2.5 Hz]
125.0
C46eC51 and 3, 4, 7, 8
1.8e1.3 m (8H, 12H)
27.0, 25.6, 25.5, 24.5, 29.3, 29.3
29.2
C45
C52
1, 2, 5, 6
3.6 t [J (H,H) ¼ 7.8 Hz]
63.0
44.2
63.1 m
3.6 t [J (H,H) ¼ 6.5 Hz]
3.4 bs 4H
Phenyl groups on P1 & P2
C14,15
C21,31,41,51
C13,20
7.6e7.2 m (12H), 7.1e7.0 (8H)
133.9 t, [J(P,C) ¼ 5.2 Hz], 128.9 d,[J(P,C) ¼ 24.4 Hz], 128.1 (t, [J(P,C) ¼ 8 Hz]
e
e
e
e
147.8 t [J(P,C) ¼ 9Hz]
137.7 dd, [J(P,C) ¼ 38.6, 2.8 Hz]
132.8 dd, [J(P,C) ¼ 31.1, 5.6 Hz]
120.7 dd, [J(P,C) ¼ 37.4, 5.3 Hz]
C9,16
a
Carbon numbers relative to the numbering scheme for 4 in Scheme 1.
Table 3
Crystal data and structure refinement parameters for 4.
Table 4
Bond lengths [Å] and angles [ꢀ] for 4.
Empirical formula
Formula weight
Crystal system
C
54H60ClIrNO4P2
C(45)-N(1)
C(9)-P(1)
C(14)-O(1)
C(15)-O(1)
C(13)-N(1)
C(1)-Ir(1)
C(2)-Ir(1)
C(5)-Ir(1)
C(6)-Ir(1)
Cl(1)-Ir(1)
Ir(1)-P(1)
1.496(12)
1.845(12)
1.405(14)
1.335(19)
1.406(17)
2.113(17)
2.105(17)
2.152(13)
2.201(12)
2.408(3)
2.387(4)
2.469(3)
1.70(3)
C(2)-Ir(1)-P(1)
C(6)-Ir(1)-P(1)
C(5)-Ir(1)-P(1)
C(2)-Ir(1)-Cl(1)
C(1)-Ir(1)-Cl(1)
C(6)-Ir(1)-Cl(1)
C(5)-Ir(1)-Cl(1)
P(1)-Ir(1)-Cl(1)
P(1)-Ir(1)-P(2)
Cl(1)-Ir(1)-P(2)
C(20)-N(1)-C(13)
C(14)-O(1)-C(15)
C(9)-P(1)-Ir(1)
C(16)-P(2)-Ir(1)
152.0(4)
81.9(5)
107.9(5)
85.0(4)
1124.68
Triclinic
P-1
Space group
a ¼ 12.4659(7) Å
b ¼ 12.6172(8) Å
c ¼ 18.2748(16) Å
Volume
a
b
g
¼ 98.836(5)ꢀ.
¼ 99.860(6)ꢀ.
¼ 111.390(4)ꢀ.
84.0(5)
155.5(4)
162.5(4)
89.11(12)
102.92(12)
87.01(11)
119.3(11)
114.7(9)
118.0(4)
117.8(4)
2562.6(3) Å3
Z
2
Density (calculated)
Absorption coefficient
F(000)
Reflections collected
Independent reflections
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
1.355 Mg/m3
2.758 mmꢁ1
1064
14,310
Ir(1)-P(2)
C(52A)-O(2A)
C(1)-Ir(1)-P(1)
11,770 [R(int) ¼ 0.1169]
113.1(5)
1.175
R1 ¼ 0.1234, wR2 ¼ 0.1924
R1 ¼ 0.2214, wR2 ¼ 0.2205
observed in the 31P NMR, indicating that the two diphosphorus
atoms were in a chemically equivalent environment.
3.2. NMR data
3.2.2. Complex 4
3.2.1. Ligand 2
The structure of complex 4 can be described as symmetrical
about a mirror plane that bisects the molecule through the
N/O/Ir/Cl vector. This renders the positions at carbons 12 & 19,
11 &18 and 10 & 17 on the nixantphos ligand chemically equivalent
as shown in Table 1. Also carbon positions on the cod ligand in
chemically equivalent positions are summarised in Table 1. The
neighbouring protons on the backbone of the coordinated ligand
exhibit slight downfield shifts in both proton and carbon NMR in
comparison to the chemical shifts of free 2 (Table 1). According to
HH (COSY), H12,19 coupled with H11,18 corresponding to a broad
doublet at 6.6 ppm (d, J(H,H) ¼ 7.8 Hz); and H11,18 coupled with
The NMR data for the free ligand prepared is presented in
Table 1. The signals at 6.4 ppm, 6.6 ppm and 6.0 ppm were assigned
to the neighbouring protons of the phenoxazine ring. The splitting
pattern and coupling constant was similar to reported nixantphos
[30]. A methylenic triplet at 3.6 and a triplet at 3.5 ppm for the
protons C46 and C52 were indicative of the functionalisation of the
amine with the n-octanol tail. The corresponding signals in the 13
NMR were at
C
d
¼ 63 and 44.7 ppm respectively. The multiplets
between 1.6 and 1.2 integrating for 12 protons were assigned to the
remaining six methylene groups. A singlet at ꢁ19.2 ppm was

8
T. Marimuthu et al. / Journal of Molecular Structure 1106 (2016) 5e9
H12,19 and H10,17 corresponding to
a
triplet at 6.8 ppm (t,
3.4. IR data
The IR analysis of complex 4 confirmed the presence of a broad
J(H,H) ¼ 7.9 Hz); and a multiplet at 6.2 ppm due to a ABCXX00
system (X and X00
¼
31P) for H10,17. In the 1H NMR spectrum, the
olefinic proton of the coordinated cod is a broad singlet at 3.4 ppm,
implying that these protons are in chemically equivalent environ-
ments. The corresponding signal appeared at 63.1 ppm in the HSQC
spectrum and lies in the region typical of bound alkene ligands. The
signal for the methylenic protons of the cod (Table 2) overlapped
with that for the methylenic protons of the alkyl chain, and the
multiplets between 1.7 and 1.2 ppm integrated for 20 protons. A
single peak was observed in the 13C NMR spectrum at 29.2 ppm
(CH2), assigned to C3, C4, C7, and C8. In the 1H NMR spectrum, a
triplet at 3.6 was assigned to the proton of the eCH2OH group of the
OH band at 3411 cmꢁ1 and a CeO absorption band near 1027 cmꢁ1
indicative of a primary alcohol. The diagnostic C]C and CH (cod)
stretching bands were at 3055, 3010, and bending vibrations 2926,
2853 and 2853 cmꢁ1 were for both methylene groups for the alkyl
tail and for the cod ligand. The aromatic CeC stretch bands (for the
phenyl ring carbon bonds) appear at 1584,1615, and 1554 cmꢁ1. The
bands for CeH bends appear around 1000 cmꢁ1 for the in-plane
bends and about 695 cmꢁ1 for the out-of-plane bend.
3.5. X-ray diffraction data
n-octanol tail and correlated with a carbon at
d 63.0 ppm in the
HSQC spectrum. Both signals showed no difference in chemical
shift, compared to the free ligand and this is indicative of the un-
coordinated O donor. The six remaining carbons on the alkyl tail
were assigned to the signals at 27.0, 25.6, 25.5, 24.5, 29.3, 29.3 ppm.
For the carbons directed bonded to phosphorus atoms, a doublet
of doublets with larger coupling constants compared to the free
ligand is observed. The largest upfield shift is found for the ipso
carbon of the phenyl and phenoxazine ring upon coordination. The
coordination shift is less pronounced on going from the ipso to para
Analysis of the single-crystal X-ray data confirmed the forma-
tion of the complex 4 and the preferred mode of coordination.
A simplified (for clarity) ORTEP representation of 4 is presented
in Fig. 2 where the coordination of ligands around Ir is described as
approximately trigonal bipyramidal when the cod ligand, the bis-
phosphino chelated ligand 2 and the Cl atom are considered. This
coordination essentially comprises of the equatorial plane and two
axial planes. The equatorial plane contains the two cis phosphines
(P1 and P2) and one cod double bond (C1]C2), while the two axial
positions contain the second cod double bond (C5]C6) and the
chloro ligand in trans positions. The PeIreP bond angle of
102.92(12)ꢀ is slightly smaller to that of [Ir(nixantphos)(cod)Cl]
carbon of the phenyl rings. For example C13,20 at
(Table 1) shifted up-field to
¼ 132.8 ppm (Table 2), whereas the
signal for the ortho carbons C21,31,41,51 only slightly moved from
d
¼ 133.2 ppm
d
d
¼ 137.7 ppm (Table 1) to
d
¼ 137.0 ppm (Table 2). Therefore the
difference in the chemical shifts between the free ligand and co-
ordinated ligand depends on the position of the carbon atom
relative to the phosphorus atom. The shielding of the aromatic
carbons is due to an increase in the
upon coordination, as the involvement of the phosphorus atom to
the phenyl ring
-delocalization is reduced by the MꢁP back
bonding. When the diphosphine ligand coordinates to the metal,
the empty d-orbitals on the phosphorus atoms accept -electron
p-electron density in the ring
p
p
density from the filled metal d-orbitals. This additional electron
density is then channelled into the phenyl ring and ligand back-
bone through the 2p-dp interactions [31,32]. This increase in
delocalization results in the observed up-field shifts of the phenyl
carbon atoms in the coordinated diphosphine. Analysis of the 31P
NMR data shows that the coordinated phosphines exhibit a slight
downfield shift (1.1 ppm) compared to the free ligand 2 suggesting
coordination. The peak was observed as a singlet at ꢁ18.1 ppm,
indicating that the two phosphines reside in similar environment,
i.e. both are equatorial as confirmed by the single crystal X-ray
analysis (see below).
3.3. Mass spectroscopy (high resolution), elemental and DSC
(differential scanning calorimeter) thermal analysis
The exact mass for 4 was calculated as 980.3332. The loss of the
labile Cl ligand results in the positively charged adduct
[Ir(cod)(NixC8OH]þ. The mass spectrum of 4 show the highest in-
tensity molecular peak at 980.3333 m/z together with several iso-
topic peaks, from 977.3170 to 983.3385 m/z with adjacent peaks
separated by 1 m/z. The observed isotope distribution pattern
shows an envelope of peaks, which is characteristic of a mono-
cation [33]. The pattern, also agrees well with the theoretical
pattern and that found for reported [Ir(cod)(nixantphos)]þ complex
[34]. The elemental analysis results (see the experimental section)
are indicative of good bulk purity of the complex. The DSC revealed
an exotherm representing crystallisation at 200 ꢀC, followed by the
melting point of 4 observed as an endotherm at 240 ꢀC. The DSC
results indicate that the complex has good thermal stability with
the onset of decomposition at temperature >400 ꢀC.
Fig. 2. ORTEP drawing of 4 showing the atom numbering scheme. Thermal ellipsoids
are shown at 50% probability with hydrogen atoms omitted for clarity.

T. Marimuthu et al. / Journal of Molecular Structure 1106 (2016) 5e9
9
Fig. 3. Crystal packing in a molecule of 4 as seen along the a axis highlighting the C(12)-H(12)/Cl(1) inter-molecular interaction.
(106.49(3)ꢀ) [34] possibly due to the strain of the alkyl tail but
comparable to [Ir(xantphos)(cod)][BArF (101.22(3)ꢀ) [15].
Considering a mean plane of planarity involving the 14 ring atoms
of the ligand 2 it can be concluded that 4 is slightly bent about an
N1eO1 axis in order to accommodate added strain due to Ir metal
coordination. The uncoordinated and related ligand 6-[4,6-
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4. Conclusions
The novel mixed donor P,O,P' iridium complex has been syn-
thesized and fully characterized. Coordination around Ir has been
confirmed by NMR and single-crystal X-ray diffraction to be
trigonal bipyramidal with both P groups of the NixC8OH bound
equatorially.
Acknowledgements
We wish to thank Dr. Manuel Fernandes (University of the
Witwatersrand) for crystal data collection, SASOL, THRIP and the
University of KwaZulu-Natal for financial support. We gratefully
acknowledge Johnson Matthey for a loan of iridium trichloride.
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Appendix A. Supplementary material
Crystallographic data for the structures in this article have been
deposited with the Cambridge Crystallographic Data Centre, CCDC
1403370. These data can be obtained free of charge at http://www.
ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road Cambridge CB2
1EZ,UK; Fax: þ44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk).
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