Conformational studies on complexes of a diimine containing a (CH 2)2 spacer: Crystal structures of a double-stranded Zn(II) meso-helicate and an enantiopure Δ-Cu(II) monohelicate

Article abstract of DOI:10.1016/j.ica.2004.02.010

Ni(II), Cu(II), Zn(II) and Cd(II) complexes of an N₄-donor Schiff base, containing (CH₂)₂ as spacer, have been prepared. The X-ray crystal structures of monohelical Ni(ETs)·H ₂O and the homochirally crystallised Δ-Cu(ETs), as well as the meso-helicate Zn₂(ETs)₂·MeCN [H₂ETs: N,N^′-bis(2-tosylaminobenzylidene)-1,2-diaminoethane] have been solved. In the latter, the ligand behaves as bis-bidentate, displaying a C -type arrangement, instead of the typical S -type fashion present in bis-helical dinuclear complexes.

Full text of DOI:10.1016/j.ica.2004.02.010

Inorganica Chimica Acta 357 (2004) 2561–2569
www.elsevier.com/locate/ica
Conformational studies on complexes of a diimine containing
a (CH2)2 spacer: crystal structures of a double-stranded Zn(II)
meso-helicate and an enantiopure D-Cu(II) monohelicate q
*
ꢀ
Ana M. Garcıa-Deibe, Jesus Sanmartın Matalobos , Matilde Fondo,
ꢀ
ꢀ
ꢀ
Miguel Vazquez, Manuel R. Bermejo
Dpto. de Quꢀımica Inorgꢀanica, Facultade de Quꢀımica, Univ. de Santiago de Compostela, Avda das Ciencias Campus Sur,
Santiago de Compostela, 15782 Spain
Received 24 October 2003; accepted 7 February 2004
Available online 6 March 2004
Abstract
Ni(II), Cu(II), Zn(II) and Cd(II) complexes of an N4-donor Schiff base, containing (CH2)2 as spacer, have been prepared. The X-ray
crystal structures of monohelical Ni(ETs) ꢀ H2O and the homochirally crystallised D-Cu(ETs), as well as the meso-helicate
Zn2(ETs)2 ꢀ MeCN [H2ETs: N,N0-bis(2-tosylaminobenzylidene)-1,2-diaminoethane] have been solved. In the latter, the ligand behaves
as bis-bidentate, displaying a ‘‘C’’-type arrangement, instead of the typical ‘‘S’’-type fashion present in bis-helical dinuclear complexes.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Supramolecular chemistry; Helicates; Schiff bases; X-ray; Spontaneous resolution; Chirality
1. Introduction
1, where AlbrechtÕs rule, as in other cases [7], is not
applicable since r ꢁ 20° (in complexes). In fact, we have
already obtained [4 + 4] Ni(II) bis-helicates with H2PTs
[3] and H2BTs [6] (Scheme 1), which contain (CH2)3 and
(CH2)4 spacers, respectively. In each one of these dou-
ble-stranded helicates, both spacers are identically
conformed, just as Scheme 1 represents, combining one
anti-C–C bond with the rest of the gauche conformed
(dd spacers for DD enantiomers, whereas KK ones dis-
play kk spacers). These conformations are also similar
to those displayed by these two free ligands in solid state
[5,8], and provide an ‘‘S’’-type arrangement, suitable for
obtaining bis-helicates.
Bis- and tris-helicates with symmetric ligands con-
taining two-membered alkyl spacers as ethylene are
common [1,2,9]. Among them, only a few spacers show
a curious gauche conformation [7d,9c,9d] instead of the
typical anti one. Even a more rigid 1,2-diaminocyclo-
hexane residue has given rise to some bis-helicates
[9a,9b,9d,10] and a meso-helicate [10]. However, when
we have used the latter cyclic alkyl spacer in N-tosyl-
substituted N4-ligands, we only could corroborate the
structure of some tetra-coordinated single helicates, as
Several supramolecular chemists have used the ligand
spacer length, flexibility, chiral substituents or template
effects to obtain some desired metallo-supramolecular
systems [1]. Among others, Albrecht [2] has studied the
transmission of chirality between two metal centres by
mechanical coupling, leading to assemble [6 + 6] tris- or
meso-helicates. He has satisfactorily explained how an
even or odd number of CH2 groups in a non-substituted
saturated alkyl spacer can lead to homochiral (KK and
DD) or heterochiral (DK) species (even ! helicates,
odd ! meso-helicates). This behaviour is restricted to
linear ligands where the lines formed by the chelating
binding sites and both ending r bonds of pure poly-
methylene spacers are parallel ðr ¼ 0Þ [2b].
In past years, we have been using some N-tosyl-
substituted diimines [3–6] of the type shown in Scheme
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.ica.2004.02.010.
* Corresponding author. Tel.: +34-981-591076; fax: +34-981-597525.
ꢀ
E-mail address: qisuso@usc.es (J. Sanmartın Matalobos).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.02.010

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2562
spectrometer, using CDCl3 or DMSO-d6 as solvents.
Infrared spectra were recorded as KBr pellets on a Bio-
Rad FTS 135 spectrophotometer in the range 4000–600
cmꢂ1. FAB mass spectra were recorded on a Micromass
Autospec spectrometer, employing m-nitrobenzyl alco-
hol as a matrix. Magnetic susceptibility measurements
were performed on pulverised samples at room tem-
perature with a Sherwood Scientific Magnetic Suscep-
tibility Balance, calibrated with mercury tetrakis
(isothiocyanato) cobaltate(II).
2.2. Complex syntheses
Complexes have been obtained by an electrochemical
method [3–6,11], in which a metal anode was oxidised in
an acetonitrile solution of H2ETs. The preparation of
Zn2(ETs)2 is outlined below. The cell can be summarised
as: ZnðþÞjH2ETs + NMe4ClO4ðMeCNÞjPtðꢂÞ
.
(Caution:
Although no problem has been encountered in this work,
all perchlorate compounds are potentially explosive, and
should be handled in small quantities and with great
care!). An acetonitrile solution (80 cm3) of H2ETs (100
mg, 0.174 mmol), containing about 30 mg of tetra-
methylammonium perchlorate, was electrolysed for 1 h
50 min, using a 5 mA current intensity and an initial
voltage of 7.1 V. Filtration and concentration of the
resulting solution yielded a solid that was washed with
diethyl ether and dried in vacuo. The ligand amount,
current intensity and electrolysis time are identical for
all the complexes. Initial voltages were in the range 7.1–
11.2 V. Cu(ETs) and Zn2(ETs)2 ꢀ MeCN crystals suitable
for X-ray studies were obtained by slow evaporation of
saturated acetonitrile solutions of the corresponding
complexes. Crystals of Ni(ETs) ꢀ H2O were obtained by
diffusion of diethyl ether into a saturated acetonitrile
solution.
Scheme 1. Schematic representation of some free ligands showing the
spacer conformations found in their crystal structures and their cor-
responding binuclear complexes. The H2ETs scheme shows the r angle
and the labelling scheme for 1H NMR studies.
occurred with a spacer derived from o-phenylenedi-
amine [11].
Here, we would like to explore how a short, but
flexible, (CH2)2 spacer can influence on the coordination
behaviour and spatial arrangement of the ligand H2ETs
[12] (‘‘C’’-type and gauche conformed in Scheme 1, just
as it has displayed in solid state) in some transition and
post-transition metal complexes.
2.2.1. Ni(ETs) ꢀ H2O
Green solid. Yield: 78%. Anal. Calc. C30H30N4NiO5S2
(649.40): C, 55.5; H, 4.6; N, 8.6; S, 9.9. Found: C, 56.0;
H, 4.3; N, 9.0; S, 9.4%. IR (KBr) m ¼ 1630 (s, C@N),
1298 (s, C–N), 1244, 1142 (s, SO2) cmꢂ1. FAB-MS:
m=z ð%Þ ¼ 631:2 (100) [Ni(ETs) + H]þ. H NMR (250
1
MHz, dmso-d6): d ¼ 2:27 (s, 6H, Hi), 3.02 (m, 2H, Hj ),
ax
eq
3.18 (m, 2H, Hj ), 7.03 (m, 4H, Hc + Hd), 7.12 (d, 4H,
Hh), 7.30 (t, 2H, He), 7.44 (d, 2H, Hf ), 7.59 (d, 4H, Hg),
7.97 (s, 2H, Hb).
2. Experimental
2.1. Materials and methods
Chemicals of the highest commercial grade available
(Aldrich) were used as received. N,N0-Bis(2-tosylamin-
obenzylidene)-1,2-diaminoethane (H2ETs) has been
prepared [12] by condensation of 2-tosylaminobenzal-
dehyde [13] with 1,2-diaminoethane. Elemental analyses
were performed on a Carlo Erba EA 1108 analyser.
NMR spectra were recorded on a Bruker DPX-250
2.2.2. Cu(ETs)
Green solid. Yield: 76%. Anal. Calc. C30H28CuN4
O4S2 (636.24): C, 56.6; H, 4.4; N, 8.8; S, 10.1. Found: C,
56.8; H, 4.0; N, 8.5; S, 9.8%. IR (KBr) m ¼ 1642 (s,
C@N), 1293 (s, C–N), 1267, 1135 (s, SO2) cmꢂ1. FAB-
MS: m=z ð%Þ ¼ 637:3 (17) [Cu(ETs) + H]þ. l (300 K):
2.2 B.M.

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2563
2.2.3. Zn2(ETs)2
structures were solved by a combination of standard
direct methods and refined by Fourier techniques [15]
based on F 2. All non-hydrogen atoms were aniso-
tropically refined, except those corresponding to the
disordered solvated water molecule of Ni(ETs) ꢀ H2O.
Hydrogen atoms were included at geometrically cal-
culated positions with thermal parameters derived from
the parent atoms, but they could not be located for the
solvated water molecule of Ni(ETs) ꢀ H2O. The final
Fourier maps were quite flat. The most significant
crystallographic data are listed in Table 1. Molecular
graphics are represented by Ortep-3 for Windows and
Platon [16].
Yellow solid. Yield: 87%. Anal. Calc. C60H56
N8O8S4Zn2 (1276.15): C, 56.5; H, 4.4; N, 8.8; S. 10.0.
Found: C, 56.1; H, 4.6; N, 9.0; S, 10.7%. IR (KBr)
m ¼ 1630 (s, C@N), 1298 (s, C–N), 1244, 1142 (s, SO2)
cmꢂ1. FAB-MS: m=z ð%Þ ¼ 637:2 (17) [Zn(ETs) + H]þ,
1275.4 (7) [Zn2(ETs)2 + H]þ. 1H NMR (250 MHz,
CDCl3) d ¼ 2:24 (s, 6H, Hi), 4.35 (m, 2H, Hj ), 4.45 (m,
ax
eq
2H, Hj ), 6.51 (d, 2H, Hc), 6.60 (t, 2H, Hd), 6.97 (d, 4H,
Hh), 7.12 (t, 2H, He), 7.32 (d, 2H, Hf ), 7.86 (d, 4H, Hg),
8.03 (s, 2H, Hb).
2.2.4. Cd(ETs)
Yellow solid. Yield: 77%. Anal. Calc. C30H28CdN4
O4S2 (685.11): C, 52.6; H, 4.1; N, 8.2; S, 9.3. Found: C,
52.3; H, 4.3; N, 7.9; S, 9.5%. IR (KBr) m ¼ 1626 (s,
C@N), 1286 (s, C–N), 1254, 1122 (s, SO2) cmꢂ1. FAB-
MS: m=z ð%Þ ¼ 686:1ð7Þ [Cd(ETs) + H]þ. 1H NMR (250
MHz, [d6]dmso) d ¼ 2:28 (s, 6H, Hi), 3.87 (s, 4H, Hj),
6.89 (t, 2H, Hd), 7.14 (m, 8H, Hc + He + Hh), 7.40 (d,
2H, Hf ), 7.81 (d, 4H, Hg), 8.56 (s, 2H, Hb).
3. Results and discussion
3.1. Single crystal X-ray diffraction studies
3.1.1. Ni(ETs) ꢀ H2O and D-Cu(ETs)
X-ray diffraction studies have allowed determining
the asymmetric unit content of Ni(ETs) ꢀ H2O, which
consists of a molecule of the neutral Ni(II) complex and
a disordered solvated water molecule. The asymmetric
unit of Cu(ETs) only includes a half molecule of this
neutral complex placed on a 2-fold axis positioned along
the y-direction of its unit cell. The whole Cu(II) complex
molecule can be generated by a symmetry operation
ðꢂx þ 2; y; z þ 2Þ and those atoms so related have been
labelled with #1. The molecular structure of Ni(ETs) is
represented in Fig. 1, while the symmetric helix corre-
sponding to D-Cu(ETs) is shown in Fig. 2. Both figures
include the atom labelling schemes used in Table 2,
which collects their most important bond distances and
angles.
2.3. Single crystal X-ray diffraction studies
Data for Ni(ETs) ꢀ H2O were collected at room
temperature using a Nicolet P3 diffractometer, whereas
a Bruker SMART CCD-1000 diffractometer was used
for Cu(ETs) and Zn2(ETs)2 ꢀ MeCN, employing in all
the cases graphite-monochromated Mo Ka radiation
ꢁ
(k ¼ 0:71073 A). No significant decay was observed
and data were corrected for Lorentz and polarisation
effects. No absorption correction was applied for
Ni(ETs) ꢀ H2O, while it was carried out semi-empiri-
cally from equivalents using the SADABS program [14]
for Cu(ETs) and Zn2(ETs)2 ꢀ MeCN. The crystal
Table 1
Crystal and refinement data for Ni(ETs) ꢀ H2O, D-Cu(ETs) and Zn2(ETs)2 ꢀ MeCN
Ni(ETs) ꢀ H2O D-Cu(ETs)
30H30N4O5NiS2
Zn2(ETs)2 ꢀ MeCN
C62H59N9O8S4Zn2
Empirical formula
M
C
C30H28CuN4O4S2
636.22
649.41
1317.24
triclinic
Crystal system
Space group
monoclinic
C2=c (No. 15)
25.1636(5)
17.5248(4)
17.0253(3)
90
monoclinic
C2 (No. 5)
16.4884(8)
7.1941(4)
13.4779(7)
90
ꢂ
P1 (No. 2)
ꢁ
a (A)
14.663(3)
14.719(3)
16.618(3)
105.758(4)
105.963(3)
106.247(4)
3065.4(11)
2
ꢁ
b (A)
ꢁ
c (A)
a (°)
b (°)
c (°)
121.256(1)
90
113.608(10)
90
3
ꢁ
V (A )
6418.2(2)
4
0.778
1464.93(13)
2
0.930
Z
l(Mo Ka) (mmꢂ1
)
0.981
8815
Reflections collected
Independent reflections ðRint
R1w2½I > 2rðIÞꢃ
13 082
4159
Þ
4195 (0.0361)
0.0660, 0.1649
0.0779, 0.1838
3304 (0.0252)
0.0452, 0.1019
0.0621, 0.1221
0.01(2)
8815 (0.0000)
0.0879, 0.1689
0.2035, 0.2165
R1w2 (all data)
Absolute structure parameter

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2564
Nimine ones, as occurs in these cases. Both coordination
environments can be described as square-planar but
tetrahedrally distorted, especially that corresponding to
the Cu(II) ion. The degree of distortion is based on h,
the dihedral angle formed by both NMN (M ¼ Ni or
Cu) terminal planes, which are ca. 15.1° and 41.1° for
Ni(II) and Cu(II) complexes, respectively. This defor-
mation is further reinforced by the half-stepped dis-
tances found for the least-squares calculated planes
corresponding to the N4 environments (maximum ca.
ꢁ
)0.19 and )0.55 A for Ni(ETs) ꢀ H2O and Cu(ETs), re-
spectively). Additionally, the nickle ion almost lies on
ꢁ
this calculated plane (deviation +0.004 A), whereas the
ꢁ
Cu(II) centre is about )0.58 A below its corresponding
plane.
3.1.2. Zn2(ETs)2 ꢀ MeCN
Fig. 1. ORTEP view of the D-Ni(ETs) helicate contained in the
asymmetric unit of Ni(ETs) ꢀ H2O. Ellipsoids are drawn at 50%
probability. H atoms and parentheses are omitted for clarity.
The asymmetric unit of this crystal structure com-
prises two half molecules of Zn2(ETs)2 and a solvated
acetonitrile molecule. The two whole complex molecules
can be symmetry generated by an inversion centre lo-
cated at the midpoint between each two metal ions (#1:
ꢂx þ 1; ꢂy þ 1; ꢂz and #2: ꢂx þ 1; ꢂy þ 1; ꢂz þ 1, for
molecules 1 and 2, respectively). Both molecules, 1 and
2, are chemically but not crystallographically equivalent,
so we will describe them jointly. The most significant
bond distances and angles are listed in Table 3. The
molecular structure of 1 is represented in Fig. 3 (another
figure corresponding to 2 has been deposited as E.S.I.).
This dinuclear Zn(II) complex is formed by two
N2 + N2 bridging ligands that tetrahedrally coordinate
both Zn(II) centres with distorted geometries. The di-
verse Zn–N distances (Table 2) are in the usual range
reported for other Zn(II) complexes containing Namide
and Nimine donor atoms [4,6,17] without following any
predetermined guideline. The value found for intramo-
ꢁ
lecular Znꢀ ꢀ ꢀZn distances (about 5.2 A) is logically
shorter than that reported for the bis-helicate
ꢁ
Zn2(PTs)2 ꢀ 1.5H2O ꢀ MeCN (ca. 6.2 A) [4]. Finally, no
relevant p–p or CH–p stacking interactions are observed
in this case.
3.2. Coordination behaviour and stereochemistry
As we could detect in other related Ni(II) complexes
[3,6,11], one of the O atoms of each tosyl group [O(2)
and O(4)] is weakly interacting with the metal centre in
Ni(ETs) ꢀ H2O, although these distances (ca. 3.0 and 2.8
Fig. 2. ORTEP plot of D-Cu(ETs). Ellipsoids are drawn at 50%
probability. H atoms and parentheses are omitted for clarity.
ꢁ
The Ni(II) and Cu(II) ions are tetra-coordinated to
the four N atoms of the Schiff base. Considering that the
ligand fulfills their stereochemical requirements, these
compounds can be considered as saturated [1c]. The M–
N distances (M ¼ Ni or Cu, Table 1) are in the usual
range reported for Ni(II) [3–6,11] and Cu(II) [4,6,11a]
complexes containing Namide and Nimine donor atoms,
where M–Namide distances are generally shorter than M–
A) are too long to be considered as true coordinated
bonds. These secondary interactions are similar to those
found in other planar Ni(II) complexes containing two-
membered cyclic spacers as 1,2-diamine-cyclohexane or
ꢁ
o-phenylenediamine residues (about 2.8 A) [11], but
substantially longer than those found in related bis-he-
licates with longer saturated alkyl spacers [3,6]. This
behaviour could be related with the well-known stereo-

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2565
Table 2
Most significant bond distances and angles for Ni(ETs) ꢀ H2O and Cu(ETs)
Ni(ETs) ꢀ H2O
Distances
Cu(ETs)
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
1.926(4)
1.870(4)
1.880(4)
1.921(4)
Cu(1)–N(1)
Cu(1)–N(2)
1.975(3)
1.954(3)
Angles
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(4)
N(2)–Ni(1)–N(3)
N(2)–Ni(1)–N(4)
N(3)–Ni(1)–N(4)
90.8(2)
168.3(2)
95.3(2)
85.1(2)
167.9(2)
90.8(2)
N(1)-Cu(1)–N(2)
N(1)–Cu(1)–N(2)#1
N(1)–Cu(1)–N(1)#1
N(2)–Cu(1)–N(2)#1
90.38(14)
148.10(14)
109.2(2)
84.8(2)
Symmetry transformations used to generate equivalent atoms. #1: ꢂx þ 2; y; ꢂz þ 2.
Table 3
Most significant bond distances and angles for Zn2(ETs)2 ꢀ MeCN
Distances
Zn(1)–N(11)
Zn(1)–N(12)
Zn(1)–N(13)
Zn(1)–N(14)
1.980(9)
Zn(2)–N(21)
Zn(2)–N(22)
Zn(2)–N(23)
Zn(2)–N(24)
1.944(9)
1.963(10)
2.006(11)
1.960(10)
2.028(10)
1.964(11)
1.969(10)
Angles
N(11)–Zn(1)–N(12)
N(11)–Zn(1)–N(13)
N(11)–Zn(1)–N(14)
N(12)–Zn(1)–N(13)
N(12)–Zn(1)–N(14)
N(13)–Zn(1)–N(14)
94.6(4)
114.3(4)
134.2(4)
110.0(5)
110.0(4)
93.4(4)
N(21)–Zn(2)–N(22)
N(21)–Zn(2)–N(23)
N(21)–Zn(2)–N(24)
N(22)–Zn(2)–N(23)
N(22)–Zn(2)–N(24)
N(23)–Zn(2)–N(24)
93.5(4)
108.8(4)
135.3(4)
108.8(4)
114.8(4)
94.6(4)
Torsion angle
N(12)–C(115)–C(116)#1–N(13)#1
55.5(19)
N(22)–C(215)–C(216)#2–N(23)#2
53.4(17)
#1: ꢂx þ 1; ꢂy þ 1; ꢂz and #2: ꢂx þ 1; ꢂy þ 1; ꢂz þ 1.
Fig. 3. ORTEP view of molecule 1 present in the asymmetric unit of Zn2(ETs)2 ꢀ MeCN. One of the ligand units is slightly shaded. Ellipsoids are
drawn at 50% probability. H atoms and parentheses are omitted for clarity.

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2566
chemical preferences of Ni(II) for square-planar or oc-
tahedral geometries [18]. Thus, we have observed
ꢁ
stronger Niꢀ ꢀ ꢀO interactions (ca. 2.5 A) in some related
bis-helicates that are bridged by longer alkyl spacers
[3,6]. These ligands seem inadequate to adopt an N4-
planar arrangement around the Ni(II) ion, so they be-
have as N2 + N2 bisbidentate with Niꢀ ꢀ ꢀO contacts,
providing distorted environments that could be de-
scribed as pseudo-octahedral N4O2 geometries.
The well-known tendency of Cu(II) to adopt tetra-
hedral or square-planar geometries gives rise to an
N4-mononucleating behaviour not only of PTs2ꢂ and
BTs2ꢂ (tetrahedral) but of ETs2ꢂ (planar). Once again,
the Cuꢀ ꢀ ꢀO(1) and Cuꢀ ꢀ ꢀO(1)#1 interactions (about 2.7
ꢁ
A) are too weak to depict this environment as pseudo-
octahedral.
As we have mentioned, the ligand behaves as bis-
bidentate in the dinuclear Zn(II) complex, but now
displaying a ‘‘C’’-type arrangement instead of the typical
‘‘S’’-type fashion present in bis-helical dinuclear com-
plexes [1,2,9d], which we have also previously found in
the [4 + 4] bis-helical Zn2(PTs)2 ꢀ 1.5H2O ꢀ MeCN [4].
ꢁ
The secondary Znꢀ ꢀ ꢀO interactions (ca. 2.7 A) compare
with those found in other related tetrahedral di- and
mononuclear zinc complexes containing (CH2)3 (ca. 2.6
ꢁ
ꢁ
A) [4] or CH2–CHOH–CH2 (ca. 2.7 A) [17] as spacers,
respectively. This seems to indicate that with a three-
membered spacer both behaviours (N4 and N2 + N2) can
satisfactorily coordinate Zn(II) ions without a signifi-
cant participation of the O atoms. However, the shorter
spacer of H2ETs, tending to provide square-planar N4-
coordination environments in mononuclear complexes,
could favour an N2 + N2-binucleating behaviour that so
favours preferred tetrahedral or even pseudooctahedral
environments [9d] around these spherical d10 metal
centres.
Fig. 4. Space-filling diagrams for D-Ni(ETs) (top) and D-Cu(ETs)
(bottom). H atoms have been omitted for clarity. The stereochemistry
of their gauche conformed ethylene spacers is indicated under their
Newman projections.
3.3. Conformation and chirality
Ni(ETs) ꢀ H2O and Cu(ETs) are intrinsically chiral
due to the helical arrangement of its chelate planes
around the metal ion, so that they present two different
isomers: K and D. Ni(ETs) ꢀ H2O crystallises as a racemic
compound, whereas Cu(ETs) has crystallised in the
chiral monoclinic C2 group. This means that all the
Cu(II) complex molecules show the same handedness in
the crystal. Refinement of the Flack enantiopole pa-
rameter [19] demonstrates that this crystal packing (de-
posited as E.S.I.) only contains the pure D-enantiomer
of the complex. Since the ligand is achiral, and conse-
quently unable to induce a predetermined chirality
[1,20], Cu(ETs) has undergone complete spontaneous
resolution upon crystallisation [21]. Whereas Cu(ETs) is
perfectly symmetric and palindromic, Ni(ETs) ꢀ H2O is
clearly non-symmetric (Fig. 4) and conformed as a ho-
mochirally crystallised Ni(II) D-monohelicate contain-
ing a related ligand but with a rigid two-membered
spacer as o-phenylene [11b]. This latter displays a curi-
ously less planar chromophore than that shown in
Ni(ETs) ꢀ H2O.
In the case of Zn2(ETs)2 ꢀ MeCN, since both tetra-
hedral metal environments are heterochiral (D and K),
and the ligands behave as helical threads around each
metal centre, the Zn(II) complex results achiral and so
can be termed as a meso-helical [2,22], box-like [10] or
side-by-side [1,23] complex, as well as a mesocate [18a].
This is one of the few crystallographically characterised
examples of double-stranded meso-helicate described
in the literature [10,22b,23] and to our knowledge,
the first example with this sort of N-tosyl-substituted
N4-diimines.

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2567
The ‘‘S’’-type fashion is typical of metallohelicates,
and these are more widespread than meso-helical ar-
chitectures, usually displaying a ‘‘C’’-type arrangement
in the ligands [1,2]. A greater steric strain in meso dia-
stereoisomers ðDKÞ compared with the corresponding
homochiral enantiomers (DD and KK) has been found
for ligands containing hardly flexible spacers and ex-
plain their scarceness in that case [2,24].
Apart from the arrangement around the metal ions as
source of chirality (D or KÞ, the two possible confor-
mations [d (right-) and k (left-handed)] of metallocycles
with ethylene diamine residues gauche conformed are
well known [25]. In the present case, Fig. 4 clearly shows
the different ligand arrangement and their ethylene
spacers conformation in each Ni(II) and Cu(II) com-
plex. The D enantiomer of the Ni(II) complex displays a
k gauche spacer conformation (Fig. 5, top), unlike the d
one adopted by D-Cu(ETs). This subtle difference be-
tween both combined conformations (D; dÞ for Cu(ETs)
and (D; k) for Ni(ETs) ꢀ H2O allows a stair-like fashion
of the latter one that could facilitate a more planar co-
ordination environment around the metal centre. This
type of spatial arrangement was also found in a peculiar
(DD; kk) Ag(I) bis-helicate with an N4-donor Schiff base
containing an ethylene spacer gauche conformed, where
the environments can be described as slightly distorted
square-planar [9c].
In the case of meso-helical Zn2(ETs)2 ꢀ MeCN, both
ethylene spacers also display opposite gauche confor-
mations (Fig. 5, middle), being so achiral as well
ðDK; dkÞ. In fact, the 10-membered central metallacycle
shows an achiral boat–chair–boat (BCB) conformation
(Fig. 5, bottom), typical of cyclodecane and its deriva-
tives [26], whereas another meso-helicate also containing
a 10-membered central ring showed a chiral twist–boat–
chair–boat (TBCB) conformation [23a].
Fig. 5. Partial ORTEP view showing the spacer conformations in: D-
Ni(ETs) (top), Zn2(ETs)2 (middle) and the BCB conformation of its
central metallacycle (bottom).
1
3.4. H NMR studies
behaviour of these methylene protons on NMR time-
scale evidences the non-planar arrangement of their
metallacycles and the chirality of the complexes [27].
Thus, Ni(ETs) ꢀ H2O and Zn2(ETs)2 seem to preserve
their helical conformation both in non-coordinating
(Cl3CD) and coordinating solvents (DMSO-d6).
Unlike the Ni(II) and Zn(II) complexes, Cd(ETs)
spectrum only shows one signal for Hj. This fact dem-
onstrates the enantiotopic nature of the geminal protons
in each methylene group on the NMR timescale. This
can be used to diagnose the planar arrangement of its
five-membered chelate ring and consequently of ETs2ꢂ
in this complex, if tosyl groups are not considered.
NOESY experiments were a key element in the full
assignment of the proton signals (Section 2) and conse-
quently, in a rigorous interpretation of the spectra cor-
responding to the Ni(II), Zn(II) and Cd(II) complexes. In
general, since Hb and Hi signals (Scheme 1) are two clear
references in the spectra, the Hi–Hh, Hb–Hc, and Hb–Hj
couplings allow an unequivocal identification of Hh, Hc,
and Hj signals, respectively. Similarly, the Hc–Hd, Hd–
He, He–Hf and Hh–Hg couplings show the respective
positions of Hd, He, Hf and Hg signals up.
The diamagnetism of the Ni(II) complex evidences the
electronic pairing in dxz, dyz, dz2 and dxy orbitals, which is
coherent with a square planar geometry for a low spin d8
ion. The observation at room temperature of two signals
for Hj is in agreement with the diastereotopic nature of
the geminal protons in the methylene groups of nickle
and zinc complexes (Fig. 5, top and middle). The AB spin
4. Conclusions
In this work, we have synthesised and X-ray
characterised three helical complexes with an

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2568
N-tosyl-substituted diimine bridged by an ethylene
group. While Zn(II) ions assume distorted tetrahedral
coordination geometries, the stereochemistry of Ni(II)
and Cu(II) can be described as tetrahedrally distorted
square-planar. The planar geometry around Cd(II) can
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