Directional control and supramolecular protection allowing the chemoand regioselective transformation of a triamine

Article abstract of DOI:10.1002/chem.200901020

A Zn^II-funnel complex based on a calix[6]arene ligand decorated with three tris(imidazolyl) arms at one end of the cone and three NH₂ substituents at the other end, acts as a multipoint recognition host for polyfunctionalized guests.

Full text of DOI:10.1002/chem.200901020

DOI: 10.1002/chem.200901020
Directional Control and Supramolecular Protection Allowing the Chemo-
and Regioselective Transformation of a Triamine
David Coquiꢀre,[a] Aurꢁlien de la Lande,[b] Olivier Parisel,[b] Thierry Prangꢁ,[c] and
Olivia Reinaud*[a]
Abstract: A ZnII-funnel complex based
on a calix[6]arene ligand decorated
with three tris(imidazolyl) arms at one
end of the cone and three NH2 sub-
stituents at the other end, acts as a
multipoint recognition host for poly-
functionalized guests. The selectivity is
ensured by coordination to ZnII, CH–p
interaction within the calix cone, and
H-bonding at both rims of the cavity.
As a result of these multiple interac-
tions, the host can wrap and orient an
unsymmetrical triamine guest with a
high selectivity. Furthermore, a proton-
monitored switch between the regio-
isomeric adducts allows reversible in-
version of the directionality of the
system. Thanks to this directional con-
trol, the regioselective mono-carba-
moylation of the unsymmetrical tri-
AHCTUNGTRENNaGUN mine guest was successfully achieved
on a preparative scale. This case study
shows that a funnel-like receptor can
be used as a supramolecular protecting
tool allowing a transformation which
would be impracticable with conven-
tional covalent chemistry.
Keywords: calixarenes · host–guest
systems · polyamines · regioselectiv-
ity · supramolecular chemistry
Introduction
also positions the substrate molecule relative to the reactive
species. As a result, a substrate presenting several sites sus-
ceptible to react, is regio- (and stereo-) selectively trans-
formed. For example, the triamine spermidine is in vivo
mono-acetylated on N1 by an N-acetyl transferase (SSAT).[1]
Achieving such a remarkable selectivity is a real challenge
for chemists. Inspired by Nature, various strategies have
been developed for addressing the question of chemo- and
regioselectivity within one specific substrate. Breslowꢀs pio-
neering work uses a cavity (a cyclodextrin) to which a reac-
tant is covalently linked. Here, the substrate binding pre-or-
ganizes the reactant relative to the reactive site. This has
been nicely illustrated by the regioselective hydroxylation of
a steroid to which two adamantane moieties have been con-
nected with an appropriate linker in order to pre-organize it
relative to an iron–porphyrin to which two cyclodextrins
have been attached.[2] An alternative strategy is to encapsu-
late the substrate with the reactant. The selectivity will then
be based on the complementary fit of both species relative
to the capsule. Such a strategy has allowed elegant transfor-
mations with high chemo- and regioselectivity.[3] One draw-
back of such an approach is, as for enzymes, restricted ap-
plicability as the substrate must simultaneously fulfil two
conditions: good host affinity and good positioning relative
to the reactant linked to the cavity. A different approach is
to trap the product, once it is formed, by a host that has a
strong affinity for the product, and a poor affinity for the
In biology, molecular recognition is a fundamental process
allowing capture and transmission of information, regula-
tion, and selective chemical transformation. This is particu-
larly true for enzymes: the access to the reactant together
with substrate binding allows efficiency and selectivity for a
given catalyzed transformation. At the active site, the
micro-environment not only guarantees specific binding but
[a] Dr. D. Coquiꢁre, Prof. O. Reinaud
Laboratoire de Chimie et Biochimie Pharmacologiques
et Toxicologiques
UMR CNRS 8601, Universitꢂ Paris Descartes
45 rue des Saints-Pꢁres, 75006 Paris (France)
Fax : (+33)142-86-83-87
E-mail: Olivia.Reinaud@parisdescartes.fr
Homepage: http://www.biomedicale.univ-paris5.fr/umr8601/
-Chimie-Bioinorganique-.html
[b] Dr. A. de la Lande, Dr. O. Parisel
Laboratoire de Chimie Thꢂorique, CNRS UMR 7616
Universitꢂ Pierre et Marie Curie (Paris 6)
4 place Jussieu, 75252 Paris cedex 05 (France)
[c] Prof. T. Prangꢂ
Laboratoire de Cristallographie et RMN Biologiques
UMR CNRS 8015, Universitꢂ Paris Descartes
4 avenue de l’Observatoire, 75006 Paris (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901020.
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FULL PAPER
substrate. This has been recently illustrated by the mono-
methylation of primary amines due to their selective binding
to a resorcinarene host.[4] The principle is versatile as it can
be applied virtually to any primary alkylamine provided it
has the adequate linker separating the amino function from
the bulky part of the molecule. Such a strategy, however,
cannot address the problem of regioselectivity as the reac-
tion proceeds far away from the cavity. Here, we present a
different strategy, based on the supramolecular protection of
a multifunctional substrate: one part of the substrate is rec-
ognized and selectively bound to a host, whereas the other
part remains free and accessible to any reactant that is
added to the solution.
plex also displayed a high propensity to interact at the large
rim aniline door with a variety of cations such as a second
metal ion,[9] a single proton or an ammonium,[10] thus giving
rise to stable tricationic structures.
In this article, we show that i) this multipoint recognition
system can wrap and orient an unsymmetrical triamine
guest with a high regioselectivity, ii) a proton-monitored
switch between the regioisomeric host–guest adducts allows
reversible inversion of the directionality of the system, and
iii) due to the open funnel-like feature of the receptor, the
calixarene–Zn host can be used as a supramolecular protect-
ing tool to direct an exogenous electrophilic reactant to a
specific guest nucleophilic N site.
Molecular recognition arises from weak but multiple non-
covalent interactions such as electrostatic, hydrogen bond-
ing, CH–p and cation–p interactions, coordination bonds, as
well as hydrophobic effects. All these interactions must be
strong enough to ensure selective binding and weak enough
to allow reversibility. We are developing biomimetic systems
where a metal-binding site is embedded at the bottom of a
hydrophobic pocket open at one side to exogenous ligands.[5]
The prototype of such receptors is a calix[6]arene function-
alized at the small rim by three imidazole arms that wrap a
Lewis acidic metal ion, leaving a single coordination site ac-
cessible for guest binding inside the funnel-like cavity
(Scheme 1).[6] Recent developments related to the large rim
functionalization of the calixarene have allowed tuning its
properties such as water solubility,[7] kinetics and thermody-
namics of the recognition process.[8] The ZnII-funnel com-
Results and Discussion
Supramolecular control of the directionality of the host–
guest adduct: The addition of one molar equivalent of N-(2-
aminoethyl)propane-1,3-diamine to a CD3CN solution of
2
complex [Zn
(1NH
)
G
(ClO4)2 led to the partial decoor-
dination of Zn2+ from the calix ligand as shown by the set
of broad resonances corresponding to free ligand 1NH
,
[9,10]
2
together with the formation of a new species (species A) de-
picted by the red trace in Figure 1. This new species displays
a C3v symmetry signature associated to one high-field reso-
nance at À0.68 ppm (b), which suggests inclusion of the tria-
mine in the calix cavity. The partial decoordination of Zn2+
is due to the binding of the triamine that acts as a strong tri-
dentate chelating ligand competing with the calix–tris(imid-
plex based on ligand 1NH (Scheme 1), in which three tBu
2
groups have been replaced by small NH2 substituents, was
shown to exhibit excellent induced-fit processes with a
cavity that expands for large guests (such as dopamine or
tryptamine) and shrinks for small ones (H2O).[8b] This com-
azole) system.[11] The progressive addition of acid (either
ACHTUNGTRENNUNG
perchloric, picric or Et3NHClO4) up to one molar equivalent
led to recoordination of Zn2+ to 1NH with concomitant
2
growing of the resonances shown in red to the detriment of
starting complex. Slight differ-
ences in the d shifts of the red
trace (e.g., b: À0.55 ppm)
denote the formation of a new
species (B) that highly resem-
bles A. After one equivalent of
added acid, species B remained
almost the only one in solu-
tion. Quite interestingly, the
presence of a single resonance
in the high-field region indi-
cates the formation of a single
regioisomer [which is the same
before (A) and after acidifica-
tion (B)]: the triamine, which
is unsymmetrical, selectively
binds the metal center by one
of its N atoms.
An in-depth NMR study of
species B (see Supporting In-
formation) revealed that the
guest is actually the mono-pro-
tonated triamine bound to
Scheme 1. Illustration of the capacity of ligand 1NH to act as a ditopic binding core.
2
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O. Reinaud et al.
steric overcrowding at the level
of the calixarene small rim if
the secondary amino group
were to coordinate the Zn
center.[6] Quite interestingly,
however, for the mono- and di-
protonated guest, a single but
different regioisomer was ob-
served: B coordinated at N8
not N1 (B’), C coordinated at
N1 not N8 (C’), respectively,[12]
(Figure 1 and Supporting Infor-
mation Figures S2–S4). The
guest positioning is thus depen-
dent on its protonation state.
The XRD structure of complex
B (displayed in Figure 2, top)
nicely illustrates the various
host–guest features involved in
the recognition process. A pri-
mary amino group is coordi-
nated to the dicationic ZnII
center that is constrained in a
tetrahedral four-coordinate en-
vironment by the calix ligand.
Two hydrogen bonds with host
Figure 1. Titration experiment showing the regioselective binding of the triamine guest into the calix funnel
and proton-induced switch of its directionality. Bottom to top: evolution of the 1H NMR spectrum (300 K,
2
250 MHz) of complex [Zn
(1NH
)
(MeCN)]
G
N-(2-aminoethyl) propane-1,3-diamine as a function of the acidity of the medium. Red trace: species A and B,
blue trace: species C, green trace: partially-protonated calix host. Legend: ~ HIm, * CH2Im, & HArNH2
and & HArtBu. Greek letters denote the position of the guest methylene groups relative to the coordinating
N atom.
Zn2+ through its N8 atom (Scheme 2). This was further con-
firmed by an XRD structure of this tricationic complex (see
below). Hence, the addition of one equivalent of acid dis-
placed the three equilibria depicted in Scheme 2 (left) in
favour of complex B.
Upon further addition of protons, a new discrete species
started to grow in (Figure 1, blue trace), followed by a third
one depicted in green to the detriment of complex B.
Whereas the green trace corresponds to the partially proton-
ated calix host with no triamine included, the blue trace is
associated to the presence of new resonances in the high-
field region (e and z, species C). This blue trace completely
disappeared when more acid was introduced, consistently
with the per-protonation of the calix ligand and full release
of free ZnII in solution. However, when a base (triethyl-
ACHTUNGTRENNUNGamine) was subsequently added to the acidic mixture, the
blue species was clearly the first one to reappear, thus being
the major host–guest adduct present in solution. Further ad-
dition of base restored species B. 2D NMR experiments (see
Supporting Information) showed that species C corresponds
to the tetracationic complex with the diprotonated triamine
bound to Zn2+ through, however, its other terminal nitrogen
(N1) (Scheme 2). All these various protonated forms of the
Zn2+ host–guest adducts of the triamine were stable over
time.
Regiospecificity: The triamine guest presents three poten-
tially coordinating atoms: two primary amines and a secon-
dary one. The observed selectivity in favor of the coordina-
tion of a primary amino moiety is readily explained by the
Scheme 2. Regioselective binding of N-(2-aminoethyl)propane-1,3-di-
G
amine to the ZnII-funnel complex based on ligand 1NH and acid–base
2
control of its directionality. (1) and (8) are arbitrary numbers used to dif-
ferentiate the two primary N atoms of the triamino guest.
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Chemo- and Regioselective Transformation of a Triamine
FULL PAPER
bonds with aniline units at the large rim thanks to an
eclipsed conformation. Such a stabilizing arrangement is in
good agreement with the XRD structure of the complex
that shows similar geometrical features when no ammoni-
um/counter-ion interaction is present. For complex B, an ad-
ditional persistent hydrogen bond between the central
amino moiety and one aniline at the large rim was observed,
which was never evidenced for isomer B’ (Figure 2). Indeed,
the calculated value of 15.4 kJmolÀ1 accounts well for one
H-bond difference.
Regioselectivity for the di-protonated triamine binding:
First, discrimination between isomers C and C’ must stem
from charge–charge repulsion within the guest, which can be
evaluated to +6 kJmolÀ1 in favor of C according to the dif-
ference in the pKa values.[14] However, the dynamics also
highlighted important differences for hydrogen bonding be-
tween the central amino group of the guest and the tris-ani-
line core. For isomer C’, two H-bonds are broken and re-
bound at a high frequency, which expresses a tension in the
structure: two aniline units are difficult to approach simulta-
neously to the guest secondary ammonium. In the C case,
two H-bonds were simultaneously persistent over time with
optimized short distances. This is further illustrated by the
H-bond lengths that exhibited a statistical distribution much
flatter for C’ than for C (Supporting Information Figures
S13, S15). The results concur well for a 20.7 kJmolÀ1 differ-
ence which was calculated for isomers C and C’.
Figure 2. Top: Side view of the XRD refined structure[13] at 100 K of one
out of the two tricationic complexes B of the asymmetric unit (the three
perchlorate anions have been omitted for clarity). The dashed lines
denote hydrogen bonding between the guest and the host. Bottom: Par-
tial views of the modeled structures of complexes B, B’, C and C’ denot-
ing the H-bonds responsible for the observed regioselectivity of binding.
In aqueous medium: When operated in an aqueous medium
(a CD3CN/D2O 1:1 mixture), the acid–base experiment
showed the clean formation of regioisomer B with, however,
no tetracationic adduct detectable (neither C nor C’). This
may result from the high degree of solvation of the free di-
protonated triamine in such a polar and protic medium,
which competes with Zn binding and calixarene inclusion. It
is, however, quite remarkable that a single regioisomer was
obtained, which is again assignable to the interaction of the
guest central N-atom with the calix large rim.
phenoxyl units further anchor this NH2 group to the bottom
of the cavity, at the level of the small rim. Interestingly, one
H-bond is much shorter than the other [dACTHUNRTGNEUNG(N,O)=2.86 vs
3.15 ꢄ], a feature which was not observed in the XRD
structure of a closely related complex obtained with heptyl-
(N,O)=2.923 and 2.925 ꢄ].[6]
ACHTUNGTRENNUNGamine as a guest [for which dHCATUNGTRENNUGN
As shown in Figure 2, this loss of symmetry might well stem
from the interaction of the triamine at the level of the large
rim. Indeed, a third, relatively strong H-bond also connects
the central N moiety of the guest to one aniline unit of the
host.
A supramolecular device for directing a chemo- and regiose-
lective transformation: The mono-functionalization of a
polyamine is classically carried out under conditions of
small conversion in order to minimize the formation of
poly-functionalized derivatives that are difficult to separate.
When unsymmetrical, the regioselective mono-transforma-
tion becomes a real challenge.[15] For example, upon reaction
with Boc2O, N-(2-aminoethyl)propane-1,3-diamine can give
rise up to seven different products, among which three are
regioisomers for the mono-reaction.[16] As a matter of fact,
isolation of a regioselectively mono-protected derivative of
this triamine has not been reported yet. The high regioselec-
tivity of the endo-binding of the triamine by our Zn complex
led us to imagine an applicative development where the
calix–Zn host would play the role of a supramolecular pro-
tection group for directing a reaction at the N1 site of the
guest (Scheme 3).
Rationalization by modeling: Free enthalpy differences be-
tween one regioisomer and the other were evaluated at
300 K (see Supporting Information). Indeed, DDG0 for re-
gioisomers B and B’ was found to be 15.4 kJmolÀ1 in favor
of B, whereas 20.7 kJmolÀ1 was obtained for species C com-
pared to C’. In both cases, these calculated values are in
agreement with the experimental results as they show
energy gaps that are high enough to account for equilibria
completely displaced towards species B or C.
Regioselectivity for the mono-protonated triamine binding:
Dynamics simulations were operated for 0.5 ns with isomers
B and B’ to obtain averaged structures. In both cases, the
terminal ammonium moiety interacts through hydrogen
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O. Reinaud et al.
recognition process has been
used for orienting the chemical
transformation of an unsym-
metrical, multifunctional sub-
strate. Indeed, very much like
in the SSAT enzyme,[1] we were
able to direct the electrophilic
reagent (Boc2O) at a single site
(N1) with an unprecedented
chemo- and regioselectivity.
The calix host allows for selec-
tive, reversible and oriented
substrate binding. It wraps one
side of the triamine, acting as a
Scheme 3. Supramolecular protection allowing the regioselective carbamoylation of N-(2-aminoethyl)propane-
1,3-diamine (Boc2O = di-tert-butyl dicarbonate). (1) and (8) are arbitrary numbers used to differentiate the
two primary N atoms of the triamino guest.
2
To
a
MeCN solution containing complex [Zn
(1NH )-
protective group, whereas the other side of the guest is ex-
posed to the reactant. In this multipoint recognition system,
i) the Zn2+ ion protects one terminal amino group by means
of a coordination link that is further stabilized by hydrogen
bonding at the oxygen-rich calixarene small rim, ii) the calix-
arene cone surrounds two amino groups out of three and
prevents their electrophilic attack by the exogenous reagent,
iii) the aniline door ensures the directionality of the guest
binding and allows the other terminal nitrogen site to stand
outside the cavity. Our preparative procedure demonstrates
that, whereas such a selective transformation cannot be ac-
complished by standard methodologies, a supramolecular
device makes it possible. We are now trying to extend this
case study and develop the applicability of such a strategy
for the selective transformation of various compounds.
A
R
ated triamine was added to produce complex B. Et3N and
Boc2O (3 and 1.2 equiv, respectively) were then carefully
added at 08C. After overnight stirring at RT, the Zn com-
plex was isolated by precipitation with Et2O and analyzed
1
by H NMR spectroscopy. The spectrum (see Supporting In-
formation) showed a quasi-quantitative conversion and the
presence of a single pure new species, a Zn complex with
guest resonances attesting to its coordination by the N8
atom and selective carbamoylation at N1. No trace of any
other regioisomer could be detected, which attests to the
very high regioselectivity of the guest transformation. From
this complex, the pure N1-monocarbamate derivative was
isolated, for the first time, in 63% yield after decomplexa-
tion with NaOH.[17] A key for the success of the procedure
was to find the right basicity of the reaction medium.
Indeed, with a smaller amount of Et3N, the reaction was
sluggish and incomplete after 24 h, which may be attributa-
ble to the low (and decreasing due to proton release as the
reaction proceeds) concentration of species A in the reac-
tion mixture; with a larger amount of Et3N, selectivity was
lost, which is related to the decoordination of ZnII from the
calixarene, as observed by NMR spectroscopy, leading to
the release of the endo-coordinated triamine. The selectivity
may also stem from the fact that among all species in equi-
librium (Scheme 2, left), species A, displaying the same re-
gioselective binding as B, is the most reactive, since ZnII
must deactivate the bound triamine in the chelate complex.
All in all, it is likely that the carbamoylation process is
under both thermodynamic and kinetic control as it depends
on the relative and evolving concentration of the different
species in equilibrium associated to their relative reactivi-
ty.[18]
Experimental Section
tert-Butyl 2-(3-aminopropylamino)ethylcarbamate: An acetonitrile solu-
tion (1m, 211 mL) containing a 1:1 mixture of N-(2-aminoethyl)propane-
2
1,3-diamine and perchloric acid was added to complex [Zn
(1NH
)N
AHCTUNTGREG(NUNN ClO4)2 (312 mg, 0.21 mmol, 1 equiv) in acetonitrile (20 mL). The forma-
tion of the complex containing the mono-protonated triamine as a guest
was followed by 1H NMR spectroscopy and the amount of added tria-
mine adjusted if necessary. Et3N (89 mL, 0.63 mmol, 3 equiv) was added
followed by the slowly careful addition of Boc2O (233 mL, 0.23 mmol,
1.1 equiv) diluted in CH3CN at 08C (over 15 min). The reaction mixture
was stirred at room temperature overnight, after which the volume of the
solution was reduced to two thirds under vacuum in order to precipitate
all the calix complex through the addition of ether.
The crude isolated solid was redissolved in CH2Cl2 (10 mL) and treated
with NaOHaq (1m, 2ꢅ5 mL). The organic phases were then dried with
Na2SO4 and evaporated, giving rise to pure ligand 1NH , ready to be recy-
2
cled. The basic aqueous phase was evaporated to dryness and extracted
with CH2Cl2 (3ꢅ5 mL). The crude oil obtained after removal of the sol-
vent under reduced pressure was purified by flash chromatography
(Alox, akt III, neutral, CH2Cl2/MeOH/NH4OH 10:10:0.1) to yield the
pure regioselectively Boc-protected triamine as an oil (29 mg, 63%).
1H NMR (400 MHz, 298 K, CDCl3): d=4.99 (brs, 1H; NHCO), 3.19 (m,
J=5.7 Hz, 2H; CH2), 2.78–2.61 (m, 6H; CH2), 1.60 (quintuplet, J=
6.9 Hz, 2H; CH2), 1.42 ppm (s, 9H; CH3); 13C NMR (100 MHz, 298 K,
CDCl3): d=156.3, 79.4, 49.4, 47.6, 40.6, 40.5, 34.0, 28.6 ppm; HRMS m/z:
calcd for C10H24N3O2: 218.1863, found 218.1863 [M+H]+.
Conclusions
In conclusion, we have shown that this host–guest adduct
behaves as a bistable system: the guest reversibly changes
from an “up” position to a “down” position depending on
the acidity of the medium, thus depicting an acid–base con-
trolled directional switch. This remarkably discriminative
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Chemo- and Regioselective Transformation of a Triamine
FULL PAPER
ionic complex hosting the diprotonated triamine with coordination
of N8 to Zn. See the Supporting Information for their schematized
and modeled structures.
Acknowledgements
This research was supported by CNRS and Agence National pour la Re-
cherche (Calixzyme) Project ANR-05-BLAN-0003). We are also grateful
to Beatriz Guimaraes and Andrew Thomson for access to the PX2a
beam line at the synchrotron SOLEIL, France.
¯
[13] Crystals are triclinic, space group P1 with a=17.492(1), b=
23.610(2), c=25.639(1) ꢄ; a=102.69(1), b=105.96(1), g=90.60(1)8
and Z=4. The structure comprises 226 non-hydrogen atoms, that
were refined against 13521 Fobs with R=8.65%. CCDC 713662
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif. See also the Supporting Information.
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N1,N4-diprotonated triamine are 10.3 and 7.5, respectively. A 1.1
pKa unit difference for the pKa2 (corresponding to DG8=
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[17] According to the 1H NMR spectrum of the crude complex after re-
action with Boc2O (and before precipitation with ether), the conver-
sion is quasi-quantitative. This attests to the sequential i) endo-bind-
ing ii) reaction of the triamine, and not the reverse. Full characteri-
zation of the Boc-triamine, either included in the calixarene cavity
or free is given in the Supporting Information.
[6] O. Sꢂnꢁque, M. N. Rager, M. Giorgi, O. Reinaud, J. Am. Chem. Soc.
2000, 122, 6183–6189.
[7] Y. Rondelez, G. Bertho, O. Reinaud, Angew. Chem. 2002, 114,
1086–1088; Angew. Chem. Int. Ed. 2002, 41, 1044–1046.
[8] a) Y. Rondelez, M.-N. Rager, A. Duprat, O. Reinaud, J. Am. Chem.
Soc. 2002, 124, 1334–1340; b) D. Coquiꢁre, J. Marrot, O. Reinaud,
Org. Biomol. Chem. 2008, 6, 3930–3934.
[18] The stoichiometric reaction of ZnII, triamine and (Boc)2O with or
without Et3N leads to an inextricable mixture of products, which
means that coordination of ZnII to the triamine out of the calix
cavity does not efficiently protect it from reacting with (Boc)2O, al-
though it certainly slows down the carbamylation.
[9] D. Coquiꢁre, J. Marrot, O. Reinaud, Chem. Commun. 2006, 3924–
3926.
[10] D. Coquiꢁre, A. de la Lande, S. Martꢆ, O. Parisel, T. Prangꢂ, O. Re-
inaud, Proc. Natl. Acad. Sci. USA 2009, 106, 10449–10454.
[11] a) R. Barbucci, L. Fabbrizzi, P. Paoletti, Inorg. Chim. Acta 1973, 7,
157–160; b) T. Itoh, Y. Fujii, T. Tada, Y. Yoshikawa, H. Hisada,
Bull. Chem. Soc. Jpn. 1996, 69, 1265–1274; c) J. Paharovꢇ, J.
ˇ
ˇ
Cernꢇk, Z. Zꢇk, J. Marek, J. Mol. Struct. 2007, 842, 117–124.
[12] B’ stands for the tricationic complex hosting the monoprotonated
triamine with coordination of N1 to Zn. C’ stands for the tetracat-
Received: April 17, 2009
Revised: June 10, 2009
Published online: September 23, 2009
Chem. Eur. J. 2009, 15, 11912 – 11917
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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