Structural and spectroscopic characterization of Cu(salen) complexes bearing long alkoxy chains

Article abstract of DOI:10.1080/00958972.2010.533762

Cu(II) salen (salen = N,N'-ethylene-bis(salicylideneimine)) complexes bearing alkoxy chains of C₁₂ (1) and C₁₀ (2) in the salicylidene moieties were synthesized and subsequently characterized by several independent methods: single cr

Full text of DOI:10.1080/00958972.2010.533762

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Structural and spectroscopic
characterization of Cu(salen)
complexes bearing long alkoxy chains
Minna T. Räisänen a , Martti Klinga a , Markku Leskelä a , Martin
Nieger a & Timo Repo a
a Laboratory of Inorganic Chemistry, Department of Chemistry,
University of Helsinki, PO Box 55, FI-00014 Helsinki, Finland
Published online: 19 Nov 2010.
To cite this article: Minna T. Räisänen , Martti Klinga , Markku Leskelä , Martin Nieger & Timo Repo
(2010): Structural and spectroscopic characterization of Cu(salen) complexes bearing long alkoxy
chains, Journal of Coordination Chemistry, 63:24, 4280-4289
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Journal of Coordination Chemistry
Vol. 63, No. 24, 20 December 2010, 4280–4289
Structural and spectroscopic characterization of Cu(salen)
complexes bearing long alkoxy chains
¨
¨
¨
MINNA T. RAISANEN*, MARTTI KLINGA, MARKKU LESKELA,
MARTIN NIEGER and TIMO REPO*
Laboratory of Inorganic Chemistry, Department of Chemistry, University of
Helsinki, PO Box 55, FI-00014 Helsinki, Finland
(Received 22 May 2010; in final form 1 October 2010)
Cu(II) salen (salen ¼ N,N0-ethylene-bis(salicylideneimine)) complexes bearing alkoxy chains of
C12 (1) and C10 (2) in the salicylidene moieties were synthesized and subsequently characterized
by several independent methods: single crystal X-ray diffraction, infrared spectroscopy, mass
spectrometry, and UV-Vis spectroscopy. The solid state structures show that the spatial
orientation and packing of the complexes are not affected by the alkyl chain length but by a
coordinated solvent molecule, as demonstrated by the structure of 1 having methanol in an
axial position.
Keywords: Copper; Salen; Solid state structure; Spectroscopy
1. Introduction
Even though Schiff-base metal complexes have been known for a long time [1], they still
evoke much current interest and are used in a variety of applications. This is partly due
to the fact that they are relatively easy to synthesize and to tune electronically and
sterically [2–9]. Schiff-base metal complexes on graphite have become important as they
provide a platform for precise metal patterned surfaces, e.g. for functional nanodevices
and for data storage applications [10]. Typically, compounds aimed for self-assembly
have long alkyl chains to provide stable 2-D structures on graphite. For instance, alkyl
chain functionalized bis-salicylaldehydato and bis-salicylaldiminato Cu(II) and Pd(II)
complexes [11, 12] as well as Co(II), Cu(II), and Ni(II) salophen (salophen ¼ N,N0-o-
phenylene-bis(salicylideneimine)) complexes [13] have been self-assembled on the
liquid–graphite interface and it has been shown that the highly-ordered 2-D patterns
formed could be controlled very precisely [11–13].
Besides for 2-D assemblies, metal Schiff-base, particularly salen (salen ¼ N,N0-
ethylene-bis(salicylideneimine)), complexes with long alkyl chains have been widely
studied as non-conventional liquid crystal compounds [6, 7, 14–25]. Thus, solid state
*Corresponding authors. Email: minna.t.raisanen@helsinki.fi; timo.repo@helsinki.fi
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.533762

Cu(salen) complexes
4281
structures of bridged Schiff-base complexes bearing long (C4–C16) alkyl chains in the
salicylidene moieties [6, 7, 14, 17, 22–27] as well as of analogous N-salicylideneimine
complexes [15, 28–30] are of interest. In this study, three solid state structures and 3-D
packings of Cu(II) salen complexes 1 (crystallized with and without a coordinated
methanol molecule) and 2 (figure 1) are presented. The effect of axially coordinated
solvent on the spatial orientation and packing of otherwise similar Cu(II)
salen-type complexes is described. The structure of 1 has been previously determined
at 153 K [23]; here the discussion is limited to structures determined at the same
temperature (173 K).
2. Experimental
2.1. General methods
1
The H-NMR spectra were recorded with a Varian Gemini 200 apparatus and the
infrared (IR) spectra (from powders) with a Perkin Elmer Spectrum One. EI and
HRMS (ESI-TOF) mass spectra were recorded with JEOL JMS-SX 102 (ionizing
Figure 1. Synthetic scheme for Cu(II) salen complexes 1–2. For details of the first reaction step, see [31].

4282
M.T. Raisanen et al.
¨ ¨
voltage 70 eV) and Bruker micrOTOF mass spectrometers, respectively. Elemental
analyses were performed with an EA 1110 CHNS-O CE instrument. All chemicals were
purchased from Aldrich and used as received. The 4-alkyloxy-2-hydroxy-benzaldehydes
were prepared according to a literature procedure [31].
2.2. Ligand synthesis
In a typical preparation of salen preligands, ethylenediamine (0.15 mL, 2.2 mmol) was
added to CHCl3 (5 mL) solution of 4-dodecyloxy-2-hydroxy-benzaldehyde (1.19 g,
3.9 mmol). The immediately formed yellow solution was stirred at room temperature
for 3 h. The solvent was removed by a rotary evaporator and N,N0-ethylene-bis(4-
dodecyloxysalicylideneimine) (A) was obtained as a yellow powder, which was purified
by washing with n-hexane. Isolated yield 0.64 g, 26%. Anal. for C40H64N2O4: 1H-NMR
(200 MHz, CDCl3, TMS): ꢀ 0.88 (t, 3JHH ¼ 6.8 Hz, 6H, CH3), 1.26 (s, 36H, CH2), 1.70–
3
1.84 (m, 4H, CH2), 3.84 (s, 4H, bridge CH2), 3.94 (t, JHH ¼ 6.5 Hz, 4H, OCH2), 6.35
3
3
(d, JHH ¼ 2.0 Hz, 2H, H–Ar), 6.39 (s, 2H, H–Ar), 7.07 (d, JHH ¼ 8.0 Hz, 2H, H–Ar),
and 8.19 (s, 2H, CH¼N) ppm. IR (cmꢀ1): ꢁ ¼ 1628 (C¼N), ꢁ ¼ 1574 (C¼C), ꢁ ¼ 1409
(C–N), and ꢁ ¼ 1285 (C–O). UV-Vis (nm): ꢂmax 389. HRMS (ESI-TOF) m/z (%):
[M þ H]þ 637 (100, calcd 637.4939. Found 637.4920, error 2.96 ppm).
N,N0-ethylene-bis(4-decyloxysalicylideneimine) (B) was obtained as a yellow powder.
1
Isolated yield 26%. Anal. for C36H56N2O4: H-NMR (200 MHz, CDCl3, TMS): ꢀ 0.88
3
(t, JHH ¼ 6.8 Hz, 6H, CH3), 1.27 (s, 28H, CH2), 1.69–1.83 (m, 4H, CH2), 3.84 (s, 4H,
3
3
bridge CH2), 3.94 (t, JHH ¼ 6.6 Hz, 4H, OCH2), 6.35 (d, JHH ¼ 2.0 Hz, 2H, H–Ar),
3
6.39 (s, 2H, H–Ar), 7.07 (d, JHH ¼ 8.0 Hz, 2H, H–Ar), and 8.19 (s, 2H, CH¼N) ppm.
IR (cmꢀ1): ꢁ ¼ 1623 (C¼N), ꢁ ¼ 1572 (C¼C), ꢁ ¼ 1410 (C–N), and ꢁ ¼ 1284 (C–O).
UV-Vis (nm): ꢂmax 388. HRMS (ESI-TOF) m/z (%): [M þ H]þ 581 (100, calcd
581.4312. Found 581.4313, error 2.1 ppm).
2.3. Complex synthesis
In a typical Cu(II) complex synthesis, ligand precursor A (0.50 g, 0.8 mmol) in CHCl3
(15 mL) was added into a MeOH solution (35 mL) of Cu(OOCCH3)2 (0.14 g, 0.8 mmol).
After 19 h of stirring at room temperature, the formed suspension was filtered and the
obtained green powder was washed with MeOH and n-hexane. Green crystals of N,N0-
ethylene-bis(4-dodecyloxysalicylideneiminato)Cu(II) (1) were obtained after recrystal-
lization from CHCl3. Isolated yield 0.42 g, 76%. Anal. Calcd for C40H62N2O4Cu (%):
C 68.78, H 8.95, and N 4.01. Found: C 68.98, H 9.35, and N 3.71. IR (cmꢀ1): ꢁ ¼ 1605
(C¼N), ꢁ ¼ 1581 (C¼C), ꢁ ¼ 1386 (C–N), and ꢁ ¼ 1302 (C–O). UV-Vis (nm): ꢂmax 350.
EIMS m/z (%): 306(10); 318(10); 637(15); and Mþ 697(100).
N,N0-ethylene-bis(4-decyloxysalicylideneiminato)Cu(II) (2) was synthesized similar to
complex 1 from compound B and Cu(OOCCH3)2. Isolated yield 78%. Anal. Calcd for
C36H54N2O4CuH2O (%): C 65.47, H 8.55, and N 4.24. Found: C 65.45, H 8.34, N 4.14.
IR (cmꢀ1): ꢁ ¼ 1608 (C¼N), ꢁ ¼ 1584 (C¼C), ꢁ ¼ 1389 (C–N), ꢁ ¼ 1308 (C–O). UV-Vis
(nm): ꢂmax 350. EIMS m/z (%): 148(17); 290(17); 580(23); Mþ 642(100).

Cu(salen) complexes
4283
Table 1. Crystallographic data and structure refinement for 1, 2, and 1 ꢁ MeOH.
Empirical Formula
Formula weight
Temperature (K)
Crystal system
Space group
C
698.46
173(2)
Monoclinic
C2/c
40H62N2O4Cu (1)
C36H54N2O4Cu (2)
642.35
173(2)
Monoclinic
C2/c
C41H66N2O5Cu (1 ꢁ MeOH)
730.50
173(2)
Monoclinic
Cc
ꢂ
˚
Unit cell dimensions (A, )
a
b
c
53.185(1)
7.398(1)
9.903(1)
91.10(1)
3895.7(7), 4
1.191
49.298(1)
7.161(1)
9.655(1)
93.51(1)
3402.0(6), 4
1.254
55.228(3)
7.649(1)
9.072(1)
94.70(1)
3819.5(7), 4
1.270
ꢃ
Volume (A ), Z
3
˚
Calculated density (Mg mꢀ3
)
Absorption coefficient (mmꢀ1
Crystal size (mm3)
R(int)
)
0.600
0.30 ꢃ 0.15 ꢃ 0.10
0.682
0.33 ꢃ 0.22 ꢃ 0.06
0.617
0.25 ꢃ 0.15 ꢃ 0.15
0.1331
0.1264
0.0340
Data/parameters
24,258/214
4471
1.036
19,683/196
3892
1.044
14,006/440
7698
1.026
R1 ¼ 0.0556,
wR2 ¼ 0.1533
0.735
Number of data refined
Goodness-of-fit on F 2
Final R indices,
R1 ¼ 0.0749,
wR2 ¼ 0.2148
1.892
R1 ¼ 0.0701,
wR2 ¼ 0.1927
1.670
Ra, wR2 [I 4 2ꢄ(I ꢀ)]3
b
˚
Residual density (e A
)
˚
˚
(1.02 A from Cu)
(0.93 A from Cu)
aR ¼ Æ ||Fo|ꢀ|Fc||/Æ |Fo|.
bwR2 ¼ ½Æ½wðFo2 ꢀ Fc2Þ ꢄ=ƽwðFc2Þ ꢄꢄ
.
2
2
1=2
2.4. Crystal structure determination
Single crystals of the complexes were selected for X-ray measurements and mounted on
a glass fiber [32]. The data were collected at 173 K using a Nonius Kappa charge-
coupled device diffractometer. The intensity data were corrected for Lorentz and
polarization effects as well as for absorption by the multi-scan method [33]. The metal
complex structures were solved with the SIR2002 program [34]. The refinements and
graphics were made using the SHELXL97 [35] and SHELXTL [36] program packages,
respectively. All non-hydrogen atoms were refined anisotropically. Hydrogens were
introduced in their calculated positions and refined with fixed geometry with respect to
their carrier atoms. In 1 ꢁ MeOH, the ethylene-bridge and the methyl-group of the
coordinated methanol are disordered. The hydrogen of the OH could not be localized
from the residual electron density map. Hydrogens of the disordered ethylene-bridge
and the methanol (also H(O) is disordered) are included using calculated positions. The
cell parameters and specific data collection parameters are presented in table 1 and
selected bond lengths and angles in table 2.
3. Results and discussion
3.1. Solid state structures of 1 and 2
Salen precursors were prepared by Schiff-base condensation between the appropriate
1,2-diamine and 4-alkyloxy-2-hydroxy-benzaldehyde. Cu(II) salen complexes 1 and 2
were synthesized in good yields from Cu(II) acetate and corresponding ligand
precursors; the obtained green powders were purified by recrystallization from

4284
M.T. Raisanen et al.
¨ ¨
ꢂ
ꢂ
˚
Table 2. Selected bond lengths (A), angles ( ), and torsion angles ( ) for 1, 2, and 1 ꢁ MeOH. The values for 1
determined in [23] are given in the brackets.
C40H62N2O4Cu
(1)
C36H54N2O4Cu
(2)
C41H66N2O5Cu
(1 ꢁ MeOH)
Cu–O
1.928(2)
[1.905(3)]
1.958(3)
[1.934(3)]
1.313(5)
1.910(2)
1.940(3)
1.295(4)
93.73(13)
1.937(3)
1.939(3)
1.960(4)
1.960(4)
1.302(6)
1.301(6)
90.61(12)
Cu–N
C¼N
O–Cu–O
92.89(14)
[92.1(2)]
O–Cu–N
94.12(11)
[94.6(1)]
161.57(12)
[161.7(1)]
84.44(17)
ꢀ102.7(4)
94.02(10)
159.87(11)
92.16(15)
92.33(15)
168.1(2)
168.2(2)
82.72(19)
91.4(9)
N–Cu–N
O–Cu–N–Cimine
84.90(15)
103.5(4)
ꢀ93.1(9)
11.2(5)
ꢀ11.9(5)
ꢀ12.8(4)
13.2(4)
N–Cu–O–CAr
0.8(3)
ꢀ83.9(4)
ꢀ0.4(3)
85.8(4)
ꢀ50.8(10)
52.3(9)
2.5(9)
O–CAr–CAr–Cimine
10.6(6)
ꢀ12.4(5)
ꢀ1.4(8)
Angle (ꢂ)
NO{Cu}/{Cu}NO
Alkoxy chain 1/alkoxy chain 2
˚
Mean sigma (A) from NO{Cu}NO plane
24.6
65
0.27
26.9
65
0.29
16.1
5
0.22
CHCl3. Crystals of 2 suitable for solid state structure determination by X-ray
crystallography were obtained by slow evaporation of CHCl3. Depending on the
solvent, either structure 1 (from CHCl3) or 1 ꢁ MeOH (from MeOH/CH2Cl2) was
obtained for 1 when the same method was employed (table 1).
The LS fits of structures 1 and 2 are similar as the average deviations are only ca
˚
0.02 A [36]. The fittings were done using the Cu1, O1, N1A, C7A, C1–C6 of 1 and the
Cu1, O1A, N1, C7, C1A–C6A of 2. A clear correlation between the alkyl chain length
˚
and the length of the a-axis can be noted, as in 1 (C12) the a-axis is 53.185 A and in 2
˚
(C10) it is 49.298 A. Complexes 1 and 2 possess crystallographic C2-symmetry and
Cu(II) has a distorted square-planar geometry typical of Cu(II) salen complexes
(figure 2) [7, 23, 37, 38]. In both structures, salen has gauche-conformation where the
carbons of the ethylene-bridge are on the opposite sides of the CuN2O2-plane.
Regardless of the length of the alkoxy substituents, the molecular packings of 1 (C12)
and 2 (C10) are similar (figure 3), in accord with the previous study [23]. In both
structures, the alternating layered structure typical of complexes with long alkyl chains
is visible: layers consisting of alkyl chains alternate with layers of salen moieties. Some
of the metal–metal distances increase with increasing alkyl chain length as expected. For
example, the distance between the metal-ions positioned upon each other in the
˚
˚
different layers is 26.84 A in 1 and 24.90 A in 2. Analogously, the distances between the
intersecting Cu ions from the different layers and planes are elongated when the alkyl
˚
˚
chain length increases: 2, 25.02–27.40 A; 1, 27.17–29.03 A (figure 3).

Cu(salen) complexes
4285
Figure 2. Molecular structures of complexes (a) 1, (b) 2, and (c) 1 ꢁ MeOH. Displacement ellipsoids appear
at the 50% probability level. Disorders in the ethylene-bridge and in the MeOH unit of 1 ꢁ MeOH as well as
hydrogens are not shown for the sake of clarity.
Figure 3. Crystal packing of 1 viewed along the angle showing the alternating structure of the alkyl chains
and the salen moieties, and Cu–Cu distances between the intersecting Cu ions from the different layers and
planes. Complex 2 is packed similar to 1. The corresponding Cu–Cu and C–C distances A–D as marked in the
˚
figure are 24.90, 8.24, 25.02–27.40, and 4.71–20.96 A.
In 1 and 2, one of the two alkyl chains points upwards from the CuN2O2-plane and
the other chain points downwards from this plane. The angle between the two planes
defined by the alkoxy chains is 65ꢂ in both structures (figure 4). The corresponding
torsion angles (table 2) are virtually the same in 1 and 2, differing only by a couple of

4286
M.T. Raisanen et al.
¨ ¨
Figure 4. Molecular structures of (a) 1 and (b) 2 showing the similarity in orientations of the alkyl chains.
Hydrogens have been omitted for the sake of clarity.
˚
degrees at maximum. The shortest Cu–Cu distances are 4.85–4.96 A and the C–C
distances between the alkyl chains are of the same order of magnitude in their packing
diagrams (figure 3).
1 ꢁ MeOH has nearly Cs-symmetry if the disorder in the ethylene-bridge is neglected.
Also, in this structure, the salen ligand has gauche-conformation. An additional axially
coordinated methanol results in square pyramidal geometry with N2O3 donors
(figure 2). The angle between the line connecting Cu and O9 of MeOH and the plane
defined by N1, N2, O1, and O2 is ca 180ꢂ. The solvent is hydrogen-bonded to the salen
˚
˚
moiety (O9ꢁ ꢁ ꢁO1 2.789 A, O9ꢁ ꢁ ꢁO2 2.784 A, symmetry operator x, 2 ꢀ y, 0.5 þ z). In
addition to the axial coordination of MeOH, a very weak Cuꢁ ꢁ ꢁH contact exists, either
˚
˚
Cu1ꢁ ꢁ ꢁH8A1–C8A 3.17 A or Cu1ꢁ ꢁ ꢁH9B2–C9B 3.18 A (due to the disorder in the
structure) with a symmetry operator x, 1 ꢀ y, ꢀ0.5 þ z.
As the Cu–O and Cu–N bond lengths in 1, 2, and 1 ꢁ MeOH (table 2) are within the
range typical for Cu(salen) complexes [7, 23, 38], MeOH in the axial position has
virtually no effect on the bond lengths around the Cu ion. Also, the C¼N bond lengths
are within a similar, characteristic range for all complexes (table 2) [39]. The distance of
˚
2.314(3) A between the Cu(II) and the axial O in 1 ꢁ MeOH is typical of Cu complexes
[23, 40–42]. Due to the MeOH guest, the crystal packing of 1 ꢁ MeOH (figure 5) differs
from that of 1 and 2 even though it shows the layered structure of alternating alkyl
˚
chains and salen moieties. In 1 ꢁ MeOH, the shortest Cu–Cu distances (5.52 A) are
significantly longer and the C–C distances between the alkyl chains considerably shorter
than for those in 1 and 2 (figures 3 and 5). In contrast to 1 and 2, both alkoxy chains in
1 ꢁ MeOH are virtually coplanar. They deviate from each other only by 5ꢂ but point in
opposite directions (figure 2c). Coordinated MeOH alters the bond and torsion angles.
The differences in the equivalent bond angles around Cu(II) and in the torsion angles
between the structures of 1 ꢁ MeOH and 1 and 2 are ca 2–8ꢂ (table 2) and even 8–34ꢂ,
respectively.
3.2. Spectroscopic characterization of 1 and 2
Infrared spectra of preligands A and B recorded at 4000–650 cmꢀ1 clearly confirm the
formation of the compounds as a strong band at 1617–1628 cmꢀ1 corresponding to
(C¼N) of the imine group is visible. In the spectra of complexes 1 and 2, the band is
shifted to 1605–1608 cmꢀ1 indicating coordination of Cu to nitrogen [43, 44]. Strong

Cu(salen) complexes
4287
Figure 5. Crystal packing of 1 ꢁ MeOH. Hydrogens and the disorders in the ethylene-bridge and MeOH
are not shown for the sake of clarity.
bands in the 793–850 cmꢀ1 region in spectra of A, B, 1, and 2 can be assigned to the out-
of-plane deformation of the aromatic rings. In the spectra of preligands, a phenolic
C–O, vibration is observed at 1284–1285 cmꢀ1, whereas in the spectra of the complexes
the band shifts to 1302–1308 cmꢀ1 and is weaker [45]. The C–N vibration shows a
characteristic 10–20 cmꢀ1 shift to lower wavenumber upon complex formation
(1409–1410 cmꢀ1 for ligands and 1386–1389 cmꢀ1 for complexes) [43].
The electronic properties of A, B, 1, and 2 were studied by UV-Vis spectroscopy at
room temperature in CHCl3 in which they are readily soluble. The electronic spectra of
A and B exhibit an absorption at 388 nm (25,773 cmꢀ1) or 389 nm (25,707 cmꢀ1) which
can be attributed to a charge transfer n ! ꢅ* band [43, 46]. Upon complexation in 1
and 2, the absorption is shifted to lower wavelength (350 nm ¼ 28571 cmꢀ1, a shoulder
around 377 nm ¼ 26525 cmꢀ1). The band at 561 nm noted in the spectra of the
complexes is assignable to d–d transitions [43].
4. Conclusion
Cu(II) Schiff-base complexes with alkyl side chains are used in various applications.
Significance of the alkyl chains and possible solvent molecules becomes evident
especially in the field of supramolecular chemistry, studied both on surfaces and in
molecular crystals. The alkyl chains and coordinated solvent molecules can have drastic
effects on 2-D surface structure formation as well as on molecular structure and
packing, as shown by several studies. Furthermore, even the slightest changes, such as
moderately differing chain length or coordinated solvent, can alter the effects. Studies
addressing these topics have the potential to provide the needed information for crystal
engineering as the design and prediction of 3-D crystals and 2-D surface structures are
far from trivial.
In this context, Cu(II) salen complexes 1 and 2, bearing long alkyl chains (C12 and
C10, respectively) in the salicylidene moieties, have been synthesized and characterized
by several other independent methods (IR, MS and UV-Vis). Of the determined solid
state structures, two are new (1 ꢁ MeOH and 2) and the structure of 1 has been
determined at a temperature different from the one in a previous study. The structures

4288
M.T. Raisanen et al.
¨ ¨
of 1 and 2, differing only by the alkyl chain length, show that the chain length has no
effect on the 3-D structures confirming the finding reported previously. An important
finding of this study is that an axially coordinated solvent can notably change the
spatial orientation and packing of the complex. This was shown by comparing the
structures of 1 and 1 ꢁ MeOH where the coordinated MeOH of the latter is the
only difference between their chemical structures. Moreover, when this result is put
in context with previous studies, it is evident that different coordinated solvent
molecules can alter the coordination sphere around Cu(II) in a different manner.
Complexes 1 and 2 show crystallographic C2-symmetry and have a distorted
square-planar coordination around Cu(II), whereas in 1 ꢁ MeOH (with nearly
Cs-symmetry) an axially coordinated solvent molecule results in square pyramidal
coordination. In all the structures, the alkyl chains and aromatic parts alternate, typical
of compounds with long alkyl chains.
Supplementary material
Crystallographic data (excluding structure factors) for the structures in this article have
been deposited with the Cambridge Crystallographic Data Centre with supplementary
publication CCDC nos 611867 (2), 611869 (1 ꢁ MeOH), and 611870 (1). Copies of the
data can be obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [Fax: þ44(0)-1223-336033; Email: deposit@ccdc.cam.ac.uk].
Acknowledgments
The financial support of the Magnus Ehrnrooth Foundation and the Mathematics and
Science Fund of the University of Helsinki are gratefully acknowledged by MTR.
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