Zinc and cadmium complexes with an achiral symmetric helicand. Crystal structure of an enantiomerically pure Λ-Zn(II) monohelicate

Article abstract of DOI:10.1039/b201433p

Zn(II) and Cd(II) complexes with an N-tosyl substituted N₄-donor Schiff base, containing a 2-propanol residue as spacer, have been prepared. The X-ray crystal structure of the monohelicate Λ-Zn(OHPTs)·H₂O [H₂OHPTs:)N,N′-bis(2-tosylaminobenzylidene)-1,3-diamino-2- propanol] has been solved. The Zn(II) ion assumes a distorted tetrahedral coordination geometry, involving the four donor N atoms of the bisdeprotonated ligand. Strong (O - H···O) hydrogen bonds between neighbouring complex and lattice water molecules lead to intricate intermolecular interactions that seem to drive the crystal packing. This Zn(II) complex shows an intense blue fluorescence in solution (λ = 430 nm, φ = 0.14), which is also observed in the solid state (λ = 490 nm). Cd(OHPTs)·4H₂O, although at a lower level (φ = 0.08), is also luminescent (λ = 430 nm) in acetonitrile solution.

Full text of DOI:10.1039/b201433p

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Zinc and cadmium complexes with an achiral symmetric helicand.
Crystal structure of an enantiomerically pure K-Zn(II) monohelicate
a
Manuel R. Bermejo,* Miguel Vazquez, Jesus Sanmartın, Ana M. Garcıa-Deibe,
a
a
a
´
´
´
´
Matilde Fondoa and Carlos Lodeirob
a
´
Dpto. de Quımica Inorganica, Facultade de Quımica, Universidade de Santiago de Compostela,
Campus Sur, Santiago de Compostela, 15782, Spain. E-mail: qisuso@usc.es
´
´
b
´
ˆ
´
Dpto de Quımica, C.Q.F.B. Facultade de Ciencias e Tecnologıa, Universidade Nova de Lisboa,
Monte de Caparica, 2859-516, Portugal
Received (in Montpellier, France) 4th February 2002, Accepted 17th June 2002
First published as an Advance Article on the web 12th September 2002
Zn(II) and Cd(II) complexes with an N-tosyl substituted N4-donor Schiff base, containing a 2-propanol residue
as spacer, have been prepared. The X-ray crystal structure of the monohelicate L-Zn(OHPTs)ꢀH2O [H2OHPTs:
N,N0-bis(2-tosylaminobenzylidene)-1,3-diamino-2-propanol] has been solved. The Zn(II) ion assumes a
distorted tetrahedral coordination geometry, involving the four donor N atoms of the bisdeprotonated ligand.
Strong (O–Hꢀ ꢀ ꢀO) hydrogen bonds between neighbouring complex and lattice water molecules lead to intricate
intermolecular interactions that seem to drive the crystal packing. This Zn(II) complex shows an intense blue
fluorescence in solution (l ¼ 430 nm, f ¼ 0.14), which is also observed in the solid state (l ¼ 490 nm).
Cd(OHPTs)ꢀ4H2O, although at a lower level (f ¼ 0.08), is also luminescent (l ¼ 430 nm) in acetonitrile
solution.
Enantiopure compounds are of central importance in many
domains of chemistry.1 The obtention of helical complexes,2,3
which are intrinsically chiral, has significantly contributed to
their development in the last years. Enantiopure ligands4 are
employed to induce stereoselectivity, or even stereospecificity,
and so provide enantiomeric excesses of the P or M metallohe-
licates.3 Achiral ligands yield racemic mixtures of self-
assembled P and M helixes, although in exceptional cases,
crystallisation allows their spontaneous resolution.5 Chroma-
tographic techniques6 or resolving agents7 can be also
employed to resolve racemates.
General construction principles involved in the preparation
of helical complexes have been widely studied.2–4,8 Now, it is
known that the spacer group between the metal binding
domains is a key element in the formation and even in the
microarchitecture of metallo-helicates. Moreover, hydrogen
bonds and other weak interactions,9 such as face-to-face or
edge-to-face p-stacking,10 are frequently responsible for the
construction of supramolecular motifs.
Scheme 1 Schematic representation of H2OHPTs and labelling
scheme for 1H NMR studies.
Experimental
In recent years, we have been working with N-tosyl substi-
tuted N4-donor diimines containing two- or three-membered
spacers.11,12 We have found that a ligand of this type contain-
ing a (CH2)3 alkyl spacer can act as a N4-tetradentate ligand
with Cd2+ and as a bis(N2-bidentate) one with Zn2+ ions,
yielding both mono- and double helical metal complexes,
respectively.11a
Here, we would like to explore how the ability of a CH2–
CH(OH)–CH2 spacer to form hydrogen bonds, can influence
the spatial arrangement of N,N0-bis(2-tosylaminobenzyli-
dene)-1,3-diamino-2-propanol (H2OHPTs, Scheme 1) in its
zinc and cadmium complexes. Additionally, the study of some
photophysical properties of the ligand and its complexes has
also attracted our attention, since another Zn(II) complex con-
taining this type of diimine, but with a 1,2-diaminocyclohexane
residue as spacer, was fluorescent.12a In this case, the presence
of a hydrogen bonding site, such as the OH group, could
enhance the fluorescence emission.13
Materials and methods
Chemicals of the highest commercial grade available (Aldrich)
were used as received. Zinc and cadmium metal plates
(Aldrich) were washed with a dilute hydrochloric acid solution
prior to electrolysis.
Elemental analyses were performed on a Carlo Erba EA
1108 analyser. NMR spectra were recorded on Bruker DPX-
250 and DRX-500 spectrometers, using acetonitrile-d3 as sol-
vent. Infrared spectra were recorded as KBr pellets on a Bio-
Rad FTS 135 spectrophotometer in the range 4000–600
cmꢁ1. Mass spectra (FAB) were recorded on a Micromass
Autospec spectrometer, employing m-nitrobenzyl alcohol as
matrix. UV absorption spectra were recorded on a Perkin–
Elmer Lambda 6 spectrophotometer, and fluorescence emis-
sion spectra on a SPEX F111 Fluorolog spectrofluorimeter.
All measurements were carried out at room temperature and
in the presence of air.
DOI: 10.1039/b201433p
New J. Chem., 2002, 26, 1365–1370
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This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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Table 1 Crystal and structure refinement data for L-Zn(OHPTs)ꢀH2O
Fluorescence emission spectra in the solid state were
recorded as KBr pellets or thin films. These were obtained
by slow evaporation of saturated compound solutions (CHCl3)
on a quartz cell. UV and fluorescence studies in solution were
carried out using 10ꢁ3–10ꢁ5 M acetonitrile solutions. Trifluoro-
acetic acid (TFA) or tetrabutylammonium hydroxide was grad-
ually added to the complex solution, and the corresponding
spectra were recorded.
Empirical formula
C31H32N4O6S2Zn
M
Crystal system
Space group
686.10
Orthorhombic
P212121
9.127(3)
16.830(3)
20.558(5)
3157.9(15)
298(2)
˚
a/A
˚
b/A
˚
c/A
3
˚
U/A
Ligand synthesis
T/K
N,N0-Bis(2-tosylaminobenzylidene)-1,3-diamino-2-propanol,
H2OHPTs, has been prepared by condensation of 2-tosyl-
aminobenzaldehyde14 (5.0 g, 18.16 mmol) with 1,3-diamino-
2-propanol (0.82 g, 9.1 mmol). A chloroform solution (150
cm3) was heated at reflux over a 4 h period and subsequently
concentrated in vacuo. The yellow precipitate formed was
collected by filtration, washed with diethyl ether (30 mL),
and dried in vacuo.
Z
4
0.959
m(Mo-Ka)/mmꢁ1
Reflections collected
Independent reflections
Rint
21160
7757
0.1065
0.0545
0.1046
0.1165
0.1208
R1 [I > 2s(I)]
wR2 [I > 2s(I)]
R1 (all data)
wR2 (all data)
H2OHPTs. Yellow solid. Yield 4.8 g (88%); mp 161 ꢂC;
Anal. found: C, 61.65; H, 5.41; N, 9.33; S, 10.48. Calcd. for
C31H32N4O5S2 : C, 61.56; H, 5.29; N, 9.27; S, 10.60%. IR
(KBr, cmꢁ1): n(O–H) 3510(b), n(N–H) 3363(b), n(C=N)
1639(s), n(C–N) 1337(s), nas(SO2) 1287(s), ns(SO2) 1159(s);
FAB MS (m/z): 605.3 (100%, M+). UV (MeCN, nm):
221(vs), 262(sh), 310(w). Fluorescence (solid state, nm):
369(vw).
˚
monochromated Mo-Ka radiation (l ¼ 0.71073 A). The struc-
ture was solved by direct methods and refined by full-matrix
least-squares based on F2, using SHELX-97 software.15 All
non-hydrogen atoms were anisotropically refined. Hydrogen
atoms were included at geometrically calculated positions with
thermal parameters derived from the parent atoms, except
those bonded to the water molecule. These could be located
in the difference map and included at these sites, with a fixed
Metal complexes
2
˚
isotropic displacement factor of U ¼ 0.1 A , without further
Caution! Although no problems have been encountered in this
work, all perchlorate compounds are potentially explosive, and
should be handled in small quantities and with great care!
Complexes have been obtained by an electrochemical
method11,12 in which a metal anode was oxidised in an aceto-
nitrile solution of H2OHPTs. The preparation of Zn(OHPTs)ꢀ
4H2O is outlined below. The cell can be summarised
as: Zn(+)|H2OHPTs + NMe4ClO4(MeCN)|Pt(ꢁ) . An acetonitrile
solution (80 cm3) of H2OHPTs (100 mg, 0.165 mmol), contain-
ing about 20 mg of tetramethylammonium perchlorate, was
electrolysed for 1.75 h using a 5 mA current intensity and a
initial voltage of 12.8 V. Filtration and concentration of the
resulting solution to a third of its initial volume yielded a
solid that was washed with diethyl ether and dried in vacuo.
The ligand amount, current intensity and electrolysis time
are identical for both M(OHPTs)ꢀ4H2O (M ¼ Zn and Cd).
Initial voltage was 9.4 V for the Cd(II) complex.
refinement. Table 1 provides a summary of crystal data, data
collection and refinement parameters. Molecular graphics are
represented by Ortep-3 for Windows.16
CCDC reference number 191646. See http://www.rsc.org/
suppdata/nj/b2/b201433p/ for crystallographic files in CIF
or other electronic format.
Results and discussion
Single crystal X-ray diffraction studies
Slow evaporation of a saturated Zn(OHPTs)ꢀ4H2O acetonitrile
solution yielded dark brown crystals of Zn(OHPTs)ꢀH2O sui-
table for X-ray diffraction studies. The crystallographic analy-
sis shows that the asymmetric unit consists of a discrete
molecule of Zn(OHPTs) and a solvated water molecule. The
molecular structure of the single-stranded (M)-helical complex
is represented in Fig. 1, with the atom labelling scheme used in
Table 2, which lists the most relevant bond distances and
angles.
Zn(OHPTs)ꢀ4H2O. Brownish solid. Yield 0.089 g (73%).
Anal. found: C, 49.87; H, 5.08; N, 7.46; S, 8.78. Calcd. for
C31H38N4O9S2Zn: C, 50.30; H, 5.18; N, 7.57; S, 8.66%; IR
(KBr, cmꢁ1): n(O–H) 3444(b), n(C=N) 1641(s), n(C–N)
1291(s), nas(SO2) 1263(s), ns(SO2) 1138(s). FAB MS (m/z):
667.3 (3%, M+ ꢁ 4H2O). UV (MeCN, nm): 232(vs), 273(sh),
347(w). Fluorescence (MeCN, nm): 430(vs), (solid state, nm)
490(vs).
The zinc(II) ion is tetra-coordinated to the four N atoms of
the Schiff base ligand and consequently, it can be considered as
saturated3a since the Schiff base fulfils its stereochemical
requirements. The Zn–N distances (Table 2) are in the usual
range reported for this type of complex.11a,17 Likewise, the
Zn–Nimine distances are slightly longer than the Zn–Namide
ones, as occurs in a related [4 + 4] bishelical Zn(II) complex.11a
One of the O atoms of each tosyl group [O(2) and O(5)] is
weakly bonded to the metal ion. These Znꢀ ꢀ ꢀO distances
Cd(OHPTs)ꢀ4H2O. Yellow solid. Yield 0.091 g (70%). Anal.
found: C, 46.97; H, 4.92; N, 7.08; S, 8.23. Calcd. for
C31H38CdN4O9S2 : C, 47.29; H, 4.87; N, 7.12; S, 8.14%; IR
(KBr, cmꢁ1): n(O–H) 3435(b), n(C=N) 1636(s), n(C–N)
1288(s), nas(SO2) 1258(s), ns(SO2) 1122(s). FAB MS (m/z):
717.1 (4%, M+ ꢁ 4H2O). UV (MeCN, nm): 237(vs), 268(sh),
341(w). Fluorescence (MeCN, nm): 430(s).
˚
(about 2.7 A) are too long to be considered as true coordinated
bonds. Thus, the coordination environment can be regarded
as pseudo-tetrahedral, as the dihedral angle y between both
N–Zn–N terminal planes is ca. 68.33ꢂ.
The central six-membered chelate ring formed by the spacer
shows a non-planar arrangement in accordance with its alkylic
nature. Its least-squares calculated plane forms angles of
49.12ꢂ and 53.01ꢂ with both terminal chelate planes, which is
indicative of a helical ligand arrangement. If only the donor
atoms are considered, the ligand does not complete a twist
Single crystal X-ray diffraction studies
The structure determination was performed at room temp-
erature on a Siemens CCD Diffractometer, using graphite-
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Fig. 2 Intermolecular O–Hꢀ ꢀ ꢀO interactions between solvated water
Fig. 1 ORTEP view of L-Zn(OHPTs). Ellipsoids are drawn at 40%
probability.
and three molecules of L-Zn(OHPTs)ꢀH2O. All of them occur in a
channel indicated by the dashed lines.
around the metal centre (ca. 260ꢂ). However, if we consider the
whole ligand, the tosyl groups slightly exceed a 360ꢂ turn, as
occurs in other related monohelicates.11a,12a The S(1)ꢀ ꢀ ꢀS(2)
Weak interactions. Many weak C–Hꢀ ꢀ ꢀO interactions were
detected and could be responsible for the non-equivalent con-
formation of the two tosyl groups in the single helicate. Like-
wise, some feeble C–H/p and p-p stacking interactions are
present as well. These interactions are the most significant in
the packing scheme5c,10 of other related crystal structures that
lack H bonding.11,12
˚
distance (ca. 4.82 A) in Zn(OHPTs)ꢀH2O is used to quantify
the helical pitch.
The OH group on the spacer of Zn(OHPTs)ꢀH2O seems to
be of the utmost importance, since the ligand without this
hydroxyl group on the spacer, PTs2ꢁ, yields a Zn(II) [4 + 4]
bishelicate.11a The explanation of this unusual behaviour is
not simple, so we have considered below some factors that
could be influential.9,10
Conformation of the spacer. Monohelical complexes contain-
ing (CH2)n spacers usually arrange their methylene chains
in rather regular gauche-only conformations.11a In
Zn(OHPTs)ꢀH2O, the presence of the OH group on the central
C atom clearly distorts the spacer (torsion angles lie in a 36–
85ꢂ range, instead of ca. 60ꢂ). The OH group is almost anti
conformed with respect to N(3) (157ꢂ) and distantly gauche
conformed with respect to N(2) (80ꢂ). This arrangement mini-
mises steric hindrances and could increase the racemisation
barrier in solution.18
Zn(HOPTs)ꢀH2O crystallises in the chiral space group
P212121 of the orthorhombic system and consequently, all
the helical molecules show the same handedness. Refinement
of the Flack enantiopole parameter19 [ꢁ0.0324(14)] demon-
strates that this crystal only contains the pure L enantiomer
of the complex. Since the ligand is achiral, and so unable to
induce a predetermined chirality,2–4 Zn(HOPTs)ꢀH2O has
undergone complete spontaneous resolution upon crystallisa-
tion.5,20 The lack of optical activity exhibited by a crystalline
sample of this Zn(II) complex in acetonitrile solution demon-
strates the presence of a racemate in solution. This behaviour
is typical of a conglomerate,20 which contains equal amounts
of crystals of both handednesses. The previous determination
of the crystal structure of anomer enantiomerically pure D
complex containing this type of Schiff base with Ni(II)12b rein-
forces the conglomerate hypothesis.
Hydrogen bonding. The OH group is interacting via H(3)
with the solvated water molecule present in the asymmetric
unit. The latter is simultaneously connected to the OH group
of a second complex molecule and to a third one, through
an O atom of one tosyl group. All of these interactions occur
along a narrow channel (Fig. 2), formed by tosyl and hydroxyl
groups of facing stacks of complex molecules that can so trap
water molecules. Hydrogen bonding could play a role in auto-
resolution.9b
Table 2 Selected bond distances and angles for L-Zn(OHPTs)ꢀH2O
Zn(1)–N(4)
Zn(1)–N(1)
1.968(3)
1.985(4)
Zn(1)–N(3)
Zn(1)–N(2)
2.035(4)
2.042(4)
N(1)–Zn(1)–N(2)
N(2)–Zn(1)–N(3)
N(3)–Zn(1)–N(4)
N(1)–Zn(1)–N(3)
N(1)–Zn(1)–N(4)
N(2)–Zn(1)–N(4)
91.26(16)
87.62(16)
91.03(15)
128.29(16)
130.17(15)
123.54(16)
N(2)–C(15)–C(16)–C(17) ꢁ37.4(6)
C(15) C(16) C(17) N(3) ꢁ36.1(6)
H bonds
1H NMR studies
H2OHPTs. A NOESY experiment shows the expected cou-
plings, those corresponding to neighbouring aromatic protons
or between the hydroxyl (Hk) and methylene (Hj) protons
(Scheme 1). This experiment also allows a full assignment of
D–Hꢀ ꢀ ꢀA
d(D–H)
0.82
0.85
d(Hꢀ ꢀ ꢀA) d(Dꢀ ꢀ ꢀA) h(DHA)
O(3)–H(3A)ꢀ ꢀ ꢀO(1S).
O(1S)–H(1SA)ꢀ ꢀ ꢀO(1)1
O(1S)–H(1SB)ꢀ ꢀ ꢀO(3)2
1.98
2.14
2.12
2.788(7) 168.2
2.968(6) 163.2
2.913(6) 156.1
0.85
1
the H NMR signals (Table 3) and indicates the close proxi-
Symmetry transformations used to generate equivalent atoms: 1x ꢁ 1,
˚
mity (below 3.2 A) between the imine (Hb), methylene (Hj)
and some aromatic (Hc and Hg) protons in solution. The
y, z; 2x ꢁ 1/2, ꢁy + 1/2, ꢁz + 2.
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was deduced from the appearance of the same absorption spec-
Table 3 1H NMR signals (in ppm) for the free ligand and complexes
using acetonitrile-d3 as solventa
trum recorded for the free ligand. The demetallation hypoth-
esis points to the reversibility of the complexation reaction,
by protonation of the amide N atoms depending on the med-
ium acidity. The observation of isosbestic points (at 225, 248,
264, 293 and 322 nm) in the UV spectra of Zn(OHPTs)ꢀH2O
during the addition of TFA suggests the existence of different
species in solution, such as H2OHPTs, Zn(OHPTs),
Zn(HOHPTs)+ or Zn(H2OHPTs)2+. Unfortunately, addi-
tional measurements to determine the H2OHPTs–Zn(II) inter-
action constant were unsuccessful, probably due to slow
complexation kinetics under these acidic conditions.
H2OHPTs presents a weak fluorescence emission in the solid
state with a maximum at 369 nm that almost disappears in acet-
onitrile or methanol solutions. This contrasts with the intense
luminescence (ca. 425 nm in MeCN solution at lexc ¼ 260
nm) shown by an equivalent Schiff base ligand containing a
(CH2)3 spacer.12a The presence of the hydroxyl group on the
spacer must be responsible for the quenching phenomenon.
The chelation of Cd(II), and especially Zn(II), to OHPTs2ꢁ
leads to an enhancement of the fluorescence emission (CHEF
effect).23 At room temperature Zn(OHPTs)ꢀH2O shows an
intense luminescence at about 490 nm in the solid state and
at 430 nm in acetonitrile solution, as its quantum yield
(f ¼ 0.14) indicates. Although the origin of the fluorescence
remains unknown, the active unit could be located on the 2-
tosylaminobenzaldehyde residue, since this exhibits an intense
luminescence at about 498 nm in MeCN solution. Fluores-
cence emission by metal complexes in the solid state presents
an attractive alternative to the usual inorganic materials
Assignment H2OHPTs
Zn(OHPTs)ꢀ4H2O Cd(OHPTs)ꢀ4H2O
Ha
Hb
13.26 s 2H
8.47 s 2H
8.48 s 1H;
8.45 s, 1H
8.46 s 2H
Hg
Hc
Hf
He
Hh
Hd
Hj
7.75 d (8.5) 4H
7.42 d (7.5) 2H
7.65 d (7.5) 2H,
7.35 t (7.5) 2H
7.25 d (8.5) 4H
7.11 t (7.5) 2H
3.99 m 2H;
7.84 d (8.5) 4H
7.47 m 4H
7.87 d (8.5) 4H
7.30 d (7.5) 2H
7.40 d (8.0) 2H
7.22 m 6H
7.28 m 6H
6.94 t (7.5) 2H
4.36 m 2H;
3.65 m 2H
4.07 m 1H
3.84 m 1H
2.35 s 6H
6.90 t (7.5) 2H
4.12 m 2H;
3.72 m 2H
4.22 m 1H
3.52 m 1H
2.33 s 6H
3.87 m 2H
4.30 b 1H
Hk
Hl
Hi
3.33 b 1H
2.31 s 6H
a
Abbreviations: s: singlet; d: doublet; t: triplet; m: multiplet; b: broad.
3J in Hz are given in parentheses.
Hb–Hg coupling reveals that H2OHPTs could be folded in a
similar conformation to that found in the solid state for a
related diimine containing a C6H10 spacer.12a
Zn(HOPTs)ꢀH2O. The presence of two signals for Hj is
coherent with the diastereotopic nature of the geminal protons
in each methylene group, as a result of the helicity.5b,21 The
methylene protons (Hj ) in an equatorial position with respect
eq
to the planes C(17)–N(3)–C(18)–C(19)–C(20) or C(15)–N(2)–
C(14)–C(13)–C(12) are observed at higher field than those sited
near the axial positions (Hj ).
ax
A NOESY experiment indicates the close proximity between
imine, methylene and hydroxyl protons as well as between Hg ,
Hf , Hj and Hl in acetonitrile solution. This last fact seems to
ax
indicate that at least one tosyl group remains nearly perpendi-
cular to the C(17)–N(3)–C(18)–C(19)–C(20) plane, in a similar
disposition to that observed in the solid state for the Zn(II) sin-
gle helicate.
COSY and NOESY experiments at room temperature
clearly show the AB spin behaviour of the methylene protons
on the NMR timescale. Thus, Zn(OHPTs)ꢀH2O does not seem
to be rapidly racemising in solution, and conglomeration20 is
the most likely procedure involved in obtaining this enantio-
pure complex.
Cd(HOPTs)ꢀ4H2O. The smaller separation between the Hj
ax
and Hj signals for Cd(OHPTs)ꢀ4H2O could indicate a more
eq
planar arrangement for OHPTs2ꢁ in the Cd(II) complex than
in the Zn(II) one. This view is reinforced by the observation
of two singlet signals for the imine protons in the Zn(II) com-
plex only, as well as the greater ionic radius of a tetracoordi-
˚
nated Cd(II) ion (0.92 A) compared to a similar Zn(II) ion
22
(0.74 A). The satellite signals allied to the main imine reso-
˚
nance in the Cd(II) complex spectrum are ascribed to coupling
between Hb and the naturally abundant nuclei of 113Cd and
111Cd (I ¼ 1/2). The intensities of the NMR satellites relative
to the central imine singlet are 1 : 6 : 1.
UV-Vis absorption and fluorescence emission studies
The stability of H2OHPTs and Zn(OHPTs)ꢀH2O in acidic acet-
onitrile solutions was studied by UV spectroscopy after addi-
tion of increasing amounts of trifluoroacetic acid (TFA). No
aqueous solvents were used due to hydrolysis and problems
with low solubility. The UV absorption of the free ligand is
unaffected even at a 1 : 12 (H2OHPTs:TFA) molar ratio. In
contrast, the complex seems to suffer a demetallation reaction
after reaching a 1 : 5 (complex:TFA) molar proportion. This
Fig. 3 Dependence of fluorescence emission intensity on added TFA
for Zn(OHPTs)ꢀH2O (in acetonitrile solution, lexc ¼ 313 nm) on the
wavelength (top) and complex : TFA molar ratio (bottom).
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H-bonded network among OH spacer groups, water molecules
and O atoms of the tosyl groups, which form channels along
the crystal.
The Zn(II) ion assumes a distorted tetrahedral coordination
geometry, involving the four N atoms of the dianionic ligand.
The N4 donor ligand displays a helical conformation, in which
the tosyl groups show a different spatial arrangement, that pre-
cludes its global consideration as a symmetric helicand thread.
Weak intramolecular C–Hꢀ ꢀ ꢀO interactions could be responsi-
ble for the unequal disposition of both tosyl groups in the sin-
gle helicate. This asymmetric helical arrangement observed in
the solid state seems to remain in acetonitrile solution.
Preliminary photophysical studies show that chelation of
Cd(II), and especially Zn(II), to OHPTs2ꢁ leads to an enhance-
ment of the fluorescence emission. A demetallation reaction of
Zn(OHPTs)ꢀH2O seems to occur in acidic acetonitrile solution
when a 1 : 5 (complex:TFA) molar ratio is reached. On the
contrary, addition of NBun4OH up to a 1 : 10 (complex:alkali)
molar ratio leads to a doubling of the emission intensity.
Fig. 4 Dependence of the fluorescence emission spectra (lexc ¼ 313
nm) for Zn(OHPTs)ꢀH2O in acetonitrile solution on the complex :
NBun4OH molar ratio.
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The variation of the fluorescence emission with TFA can be
observed in Fig. 3. The addition of acid up to equimolar
amounts of complex and TFA slightly increases the fluores-
cence intensity. Nevertheless, after this maximum, the succes-
sive addition of acid leads to a progressive decrease of the
fluorescence that finally (above a 1 : 5 molar ratio) results in
2
3
4
5
a
residual emission. In the case of Cd(OHPTs)ꢀ4H2O
(f ¼ 0.08) the fluorescence emission decreases to half intensity
when a 1 : 1.5 complex:TFA molar ratio is reached, and it is
totally quenched after increasing the molar ratio up to 1 : 5,
suggesting a stability similar to that observed for the zinc com-
plex in acid media.
6
7
8
9
Furthermore, we have studied the stability of Zn(OHPTs)ꢀ
H2O in alkaline solutions, when mixed with tetrabutylammo-
nium hydroxide in 1 : 5, 1 : 10, 1 : 15, 1 : 25 and 1 : 30 ratios.
UV absorption spectra are highly dependent on the amount of
NBun4OH added. This study was monitored by NOESY and
COSY experiments for Zn(OHPTs)ꢀH2O:ND4OD mixtures
with the mentioned ratios. Contrary to what was observed dur-
ing the addition of TFA, addition of NBun4OH leads to a gra-
dual shift to longer wavelength of the three characteristic UV
bands. The fluorescence emission band experiences a notable
intensity increase and a red-shift, after addition of NBun4OH,
reaching a maximum with a 1 : 10 molar ratio (476 nm). The
successive addition of the base until 1 : 15, 1 : 25 and finally
1 : 30 (complex:NBun4OH) molar ratios, results in a gradual
fluorescence quenching, which Fig. 4 illustrates. However, this
1 : 30 mixture is still more fluorescent than the Zn(II) complex
solution.
10 (a) N. Yoshida, H. Oshio and T. Ito, J. Chem. Soc., Perkin Trans.,
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´
11 (a) M. Vazquez, M. R. Bermejo, M. Fondo, A. M. Garcıa-Deibe,
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´
´
´
´
(b) M. Vazquez, M. R. Bermejo, M. Fondo, A. M. Gonzalez, J.
Mahıa, L. Sorace and D. Gatteschi, Eur. J. Inorg. Chem., 2001,
The simultaneous 1H NMR experiments seem to indicate
that after reaching a 1 : 5 molar ratio, the complex remains
unaltered in solution. However, the spectra corresponding to
a 1 : 10 mixture show some significant modifications, related
to the appearance of new species that show an upfield shift
of their aromatic signals with respect to the Zn(OHPTs)ꢀH2O
spectrum. Their intensity and number progressively increase
until reaching the 1 : 30 ratio, whilst the Zn(II) complex signals
are still mainly observable.
´
1863.
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´
Deibe, C. Lodeiro and J. Mahıa, J. Chem. Soc., Dalton Trans.,
´
´
´
2002, 870; (b) M. Vazquez, M. R. Bermejo, M. Fondo, A. M. Gar-
´
cıa-Deibe, J. Sanmartın, A. M. Gonzalez and R. Pedrido,
´ ´
Z. Anorg. Allg. Chem., 2002, 628, 1068.
´
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´
14 J. Mahıa, M. Maestro, M. Vazquez, M. R. Bermejo, A. M. Gon-
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´
Conclusions
´
15 G. M. Sheldrick, SHELX-97 (SHELXS 97 and SHELXL 97),
Programs for Crystal Structure Analyses, University of Go¨ttingen,
Germany, 1998.
Crystallisation of Zn(OHPTs)ꢀ4H2O occurs with spontaneous
resolution to produce enantiopure crystals of L-
Zn(OHPTs)ꢀH2O. The crystal packing is based on an intricate
16 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
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