Metal-directed assembly of chiral bis-Zn(ii) Schiff base structures

Article abstract of DOI:10.1039/c2dt30642e

Tetra-Schiff bases derived from (chiral) bis-salphen ligand scaffolds furnish, upon metalation with appropriate metal reagents, their multinuclear structures with associated Zn(OAc)₂ or Zn(OH)₂ fragments. The tendency of retaining these salts was investigated using four different (chiral) bis-salphen scaffolds. The presence of the additional Zn ions was supported by NMR studies, mass determinations and X-ray crystallography showing in two cases the possible mode of coordination within these multinuclear structures. In one case, dimerization of the Zn₃ complex leads to a unique hexanuclear Zn₆ complex being a mixture of diastereoisomeric complexes as revealed by NMR spectroscopy.

Full text of DOI:10.1039/c2dt30642e

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 9766
www.rsc.org/dalton
PAPER
Metal-directed assembly of chiral bis-Zn(II) Schiff base structures†
Martha V. Escárcega-Bobadilla,a Daniele Anselmo,a Sander J. Wezenberg,a Eduardo C. Escudero-Adán,a
Marta Martínez Belmonte,a Eddy Martina and Arjan W. Kleij*a,b
Received 21st March 2012, Accepted 26th June 2012
DOI: 10.1039/c2dt30642e
Tetra-Schiff bases derived from (chiral) bis-salphen ligand scaffolds furnish, upon metalation with
appropriate metal reagents, their multinuclear structures with associated Zn(OAc)2 or Zn(OH)2 fragments.
The tendency of retaining these salts was investigated using four different (chiral) bis-salphen scaffolds.
The presence of the additional Zn ions was supported by NMR studies, mass determinations and X-ray
crystallography showing in two cases the possible mode of coordination within these multinuclear
structures. In one case, dimerization of the Zn3 complex leads to a unique hexanuclear Zn6 complex being
a mixture of diastereoisomeric complexes as revealed by NMR spectroscopy.
Introduction
Schiff base structures find use in numerous areas of academic
research including the design and synthesis of magnetic
materials,1 homogeneous catalysis,2 multinuclear (cluster) com-
pounds with interesting photophysical and/or electronic fea-
tures,3 supramolecular chemistry,4 and sensing devices.5 One
major advantage of using Schiff base systems is their relative
easy synthesis that stems from the well-known condensation
Fig. 1 A bis-Zn(salphen) complex with an associated HOAc molecule
reaction between a ketone or aldehyde combined with an amine
reagent. The synthesis of compounds comprising multiple imine
(red and blue fragments) in an unusual intramolecular binding mode.10
bonds within the targeted structure has the intrinsic difficulty
that isolation and purification may be hampered by the reversible
nature of the imine bond.6 Therefore, a solution often used to
creation of supramolecular chirality in this host system is the
exchange of a formally associated HOAc molecule (Fig. 1) for
chiral acids. This HOAc originates from the synthesis of the host
that involves metalation of the in situ formed bis-salphen ligand
by zinc acetate dihydrate; the isolated product contains one
molecule of HOAc of which the unusual binding was confirmed
by MS, X-ray crystallography and various 1D and 2D NMR
spectroscopic methods. This HOAc molecule is in fact split into
a bridging acetate binding to both Zn(II) centers of the host mo-
lecules, whereas the residual proton is involved in hydrogen
binding to the interior phenolic O-atoms. This structural motif
was supported (in part) by X-ray diffraction studies, and a
detailed DFT analysis revealed the preferential location of the
proton. The intramolecularly associated HOAc can be regarded
as a directing agent to yield the bis-metalated structure after
work-up. Of particular note is the fact that the bis-salphen scaf-
fold could not be formed and/or isolated in the absence of the
Zn salt and thus the latter is also a prerequisite to access these
kinds of host systems.
prevent undesired imine hydrolysis or equilibration of distinct
imine bonds within the same Schiff base framework is the use of
metal templation or stable ketimine intermediates.7 The imine
coordination to metal ions indeed has proven to be a highly
effective tool for the selective isolation of, for instance, macro-
cyclic8 and other types of multinuclear Schiff base complexes.9
As part of our ongoing activities with multinuclear salphen
[salphen = N,N′-phenylene-1,2-bis(salicylimine)] structures, we
recently reported on the use of biphenyl-bridged bis-Zn(salphen)s.10
These complexes have two distinct chiral conformations (with
P- or M-helicity) that interconvert in the absence of strongly
coordinating ditopic ligands. The use of these systems as hosts
for chiral carboxylic acids in supramolecular chirogenesis was
effectively demonstrated. The operative mechanism in the
aInstitute of Chemical Research of Catalonia (ICIQ), Av. Països
Catalans 16, 43007 Tarragona, Spain. E-mail: akleij@iciq.es;
Fax: +34 977920224; Tel: +34 977920247
bCatalan Institute for Research and Advanced Studies (ICREA),
Pg. Lluís Companys 23, 08010 Barcelona, Spain
We were wondering whether this directing effect is of a more
general nature since more effective host systems for chirogenesis
applications are foreseen that do not incorporate such salts that
likely compete for substrate binding.11 Hence we have investi-
gated the formation of structurally analogous (chiral) bis-Zn-
†Electronic supplementary information (ESI) available: Further analyti-
cal details, copies of relevant electronic spectra and additional X-ray/MS
details. CCDC 871926 and 871927. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt30642e
9766 | Dalton Trans., 2012, 41, 9766–9772
This journal is © The Royal Society of Chemistry 2012

View Online
(salphen)s under comparable reaction conditions. Our new
results have revealed that, unlike for the previously reported
system,10 these structures comprise one molecule of the metal
reagent (rather than HOAc) once isolated and their inclusion is
facilitated through Zn–O coordination motifs. The interaction of
the Zn(II) salt in these new structures has been investigated in
detail using 1D and 2D NMR, DOSY, mass spectrometry, and
X-ray diffraction studies. A possible explanation for the observed
differences in these template reactions is put forward, and the
data should allow for more efficient designs of supramolecular
host systems with interesting binding properties for a wide range
of ditopic ligands useful in chirality sensing applications.
ligand scaffold (see Scheme 1). In order to see whether other
ligand scaffolds (L2 and L3) would generate similar trinuclear
compositions, we also used the chiral reagent (S)-2,2′-dihy-
droxy-[1,1′-binaphthalene]-3,3′-dialdehyde and reacted this with
mono-imines A and B in the presence of a slight excess of
Zn(OAc)2; this afforded the trinuclear complexes [Zn3L2-
(OAc)2] 2 and [Zn3L3(OAc)2] 3 in 43% and 61% isolated yield,
respectively.
The 1H NMR spectra of [Zn3L1(OAc)2] 1, [Zn3L2(OAc)2]
2 and [Zn3L3(OAc)2] 3 recorded in either d6-DMSO or
d6-acetone show the presence of a characteristic singlet resonance
at δ ∼ 1.80 ppm (in d6-DMSO) corresponding to six protons
(OAc fragments) by signal integration compared to four CHvN
protons of the respective ligand scaffold. Note that “free” un-
coordinated Zn(OAc)2 also provides a peak at 1.80 ppm in this
deuterated solvent. Therefore, one can conclude that under these
polar conditions these supramolecular assemblies dissociate into
the individual components.
Furthermore, in the MALDI(+) mass spectra, clear evidence
for the formation of these Zn(OAc)2-associated species was
found as in all three cases a molecular ion peak was observed.
As an illustrative example, Fig. 2 displays the observed and
calculated isotopic pattern for [Zn3L2(OAc)2] 2; as can be
clearly noted, the presence of multiple Zn ions in the structure
provides a spectrometric signature that is easily recognized and
therefore allows, in combination with the NMR data, fast
identification of the molecular formula.
Although the molecular composition of complexes [Zn3L-
(OAc)2] 1–3 could thus be easily established, the possible
coordination mode of the Zn(OAc)2 to the Zn(II) centers in these
bis-Zn(salphen)s could not be easily deduced with these data.
Fortunately, we were able to grow single crystalline material for
trinuclear [Zn3L1(OAc)2] 1 in the presence of hexylamine, and
the structure that was determined by X-ray crystallography is
shown in Fig. 3. The structure comprises the bis-Zn(salphen)
complex that is axially coordinated on one side by a hexylamine,
Results and discussion
The reaction conditions for the construction of bis-Zn(salphen)s
of the type shown in Fig. 1 allow for direct synthesis of the
metalated bis-salphen structures by treatment of bis-aldehydes
with appropriate mono-imine (half-salphen) reagents (A and B,
Scheme 1) in the presence of Zn(OAc)2. In order to assess
whether the unique binding of HOAc is also observed for other,
differently substituted bis-Zn(salphen)s we first used mono-
imine reagent A to prepare complex 1. The only difference of
this complex (1) with the previously reported HOAc-containing
species (Fig. 1) is the presence of two additional tBu groups on
the bis-salphen scaffold. Complex 1 was isolated as an orange
solid (47%) and was first analyzed by NMR spectroscopy and
MALDI(+) mass spectrometry. Interestingly, the compound did
not contain an associated HOAc but instead one molecule of
Zn(OAc)2 (presumably coordinating to the other Zn centers via
bridging acetates). The molecular formula can thus be abbre-
viated as [Zn3L1(OAc)2] where L1 stands for the bis-salphen
Fig. 2 Observed and calculated isotopic pattern for [M+] related to the
molecular formula C70H72N4O10Zn3; this formula corresponds to the
proposed composition for complex [Zn3L2(OAc)2] 2 depicted in
Scheme 1.
Scheme 1 Synthesis of trinuclear complexes [Zn3L(OAc)2] 1–3 where
L = L1, L2 or L3.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9766–9772 | 9767

View Online
Fig. 3 X-ray molecular structure (ellipsoids drawn at 30% probability for clarity) determined for 1 upon crystallization in the presence of hexyl-
amine. H-atoms and co-crystallized solvent molecules are omitted for clarity as well as part of the disorder in one of the hexylamines and tBu groups.
Selected bond lengths (Å) and angles (°), esd’s in parentheses: Zn(2)–O(3) = 1.976(3), Zn(2)–O(4) = 1.971(5), Zn(2)–O(2B) = 2.044(4), Zn(2)–N(3) =
2.090(6), Zn(2)–N(4) = 2.081(4), Zn(3)–N(1C) = 2.015(5), Zn(3)–N(1D) = 2.038(7), Zn(3)–O(1B) = 2.004(4), Zn(3)–O(1E) = 1.973(4); O(4)–Zn(2)–
O(3) = 97.77(17), N(4)–Zn(2)–N(3) = 77.5(2), N(4)–Zn(2)–O(2B) = 104.54(16), O(3)–Zn(2)–O(2B) = 93.17(14), N(1C)–Zn(3)–N(1D) = 108.4(2),
O(1B)–Zn(3)–O(1E) = 93.79(15).
different Zn reagent (ZnEt2); this should prevent any inclusion
of a Zn salt after isolation of the product.11 Thus the bis-salphen
ligand precursor L4-H4 was treated with ZnEt2 in hexane result-
ing in immediate precipitation of an orange solid. Analysis of
this product (i.e., complex [Zn3L4(OH)2]2 4) was done by NMR
spectroscopy and MALDI(+)-MS. Interestingly, the 1H NMR
spectrum recorded for [Zn3L4(OH)2]2 4 in CD2Cl2 showed the
presence of two separate signals for the imine-H at δ = 8.08
and 8.79 ppm respectively. More surprisingly, two high-field
singlet resonances were observed at δ = −0.64 and 0.12 ppm
that were tentatively assigned to OH groups (Fig. 5). The
13C{1H} also showed the presence of a more complicated
species (see ESI†).
In order to elucidate the identity of the product isolated and to
compare the analytical data with those gathered for complexes
1–3, X-ray diffraction studies were undertaken (Fig. 4). The
structure of [Zn3L4(OH)2]2 4 comprises a hexanuclear Zn6
complex having two bis-Zn(salphen)s interconnected through
two additional Zn ions and four hydroxo (OH) bridging
Scheme 2 Synthesis of hexanuclear complex [Zn3L4(OH)2]2 4 from
precursor ligand L4-H4.
ligands. The molecule therefore could be regarded as a “bis-Zn-
(OH)2” coordination complex. We believe that the Zn(OH)2
units originate from the synthetic procedure and likely is the
result of partial hydrolysis of the ZnEt2 before and/or during
work-up and subsequent combination with the product upon
precipitation.
In each crystallographically independent unit, each Zn ion [for
example, Zn(1) in Fig. 4] present in the salphen coordination
pocket is surrounded by the tetradentate N2O2 ligand and axially
coordinated by an OH anion [O(6)] which in turn bridges to an
“external” Zn center [Zn(3)]. This Zn(3) is coordinated by three
salphen O-atoms from the bis-salphen scaffold (two from one
while the other Zn ion is coordinated by a bridging acetate. The
latter is also connected to a third Zn center that has two hexyl-
amine ligands and a monodentate OAc anion associated.
Although the structure shows that the Zn(OAc)2 unit is here
not acting as an intramolecular bridge between the two
Zn(salphen)s, under relatively non-polar conditions such a brid-
ging nature cannot be ruled out. Also, the presence of a compet-
ing amine can affect a possible bidentate and/or bridging mode.
Then, a different bis-salphen scaffold L4-H4 (Scheme 2) was
probed which contains two ketimine fragments that allow for iso-
lation of the free base ligand and subsequent metalation with a
9768 | Dalton Trans., 2012, 41, 9766–9772
This journal is © The Royal Society of Chemistry 2012

View Online
Fig. 4 X-ray molecular structure (ellipsoids drawn at 50% probability) determined for 4; H-atoms and co-crystallized solvent molecules are omitted
for clarity and only a partial numbering scheme is presented. The disorder in the co-crystallized solvent molecules (DCM and DCE) is not shown
here. Selected bond lengths (Å) and angles (°), esd’s in parentheses: Zn(1)–O(1) = 1.9445(17), Zn(1)–(O2) = 2.0963(15), Zn(1)–O(6) = 1.9573(17),
Zn(1)–N(1) = 2.1613(19), Zn(1)–N(2) = 2.097(2), Zn(3)–O(4) = 2.0682(16), Zn(3)–O(6) = 1.9934(16), Zn(3)–O(2) = 2.1375(16), Zn(3)–O(3) =
2.0813(16), Zn(3)–O(5) = 1.9492(16); N(1)–Zn(1)–N(2) = 76.91(7), O(1)–Zn(1)–O(2) = 99.06(7), N(1)–Zn(1)–O(1) = 88.64(7), N(1)–Zn(1)–O(2) =
157.28(7), N(1)–Zn(1)–O(6) = 116.42(7), O(4)–Zn(3)–O(6) = 97.95(7), O(3)–Zn(3)–O(6) = 145.85(7), O(2)–Zn(3)–O(6) = 79.89(6).
Obviously the bis-Zn(salphen) complex exists as a racemate,
though in the solid state we observed only one of the possible
diastereoisomers with both biphenyl fragments having M-helicity
(or R-configuration). A comparison between the solid state
features and the NMR data for [Zn3L4(OH)2]2 4 reveals that the
assembly is stable in solution as two sets of resonances are
noted (Fig. 5). The presence of two distinct high-field singlets at
δ = −0.64 and 0.12 ppm (vide supra, and Fig. 5b) ascribed to
the OH anions and two imine-H at δ = 8.79 and 8.08 ppm is
thus consistent with the fact that the hexanuclear assembly is
retained in solution as the individual components would give
rise to only one observable imine and one OH resonance.
Further evidence for this was provided by a DOSY NMR experi-
ment (ESI†) carried out in CD2Cl2: the hydrodynamic diameter
that was calculated (∼14 Å) reasonably fits with the minimal
diameter of around 12 Å of the hexanuclear aggregate [Zn3L4-
(OH)2]2 4 as observed in the solid state (Fig. 4). Note that an
Fig. 5 Selected NMR regions (CD2Cl2) for complex [Zn3L4(OH)2]2 4
exact fit is not feasible due to the ellipsoidal nature of the
and partial peak assignment: (a) aromatic region (5–9 ppm) and (b) ali-
complex.
phatic region (−1 to −0.5 ppm). The asterisk denotes the residual
solvent signal.
In order to understand which factors control the formation of
the multinuclear Zn-complexes 1–4 versus the HOAc-associated
bis-Zn(salphen)s reported previously (Fig. 1),10 we closely
examined the reaction stoichiometry. In all the present examples
(complexes 1–4) a (slight) excess of Zn reagent was used
whereas the HOAc-containing complex (Fig. 1) was prepared in
the presence of a slight deficit of Zn(OAc)2. This difference
could therefore be decisive in the formation of either the HOAc-
or the Zn(OAc)2-associated containing structures. To further test
salphen and one from another salphen fragment) and a second
OH anion coordinated to Zn(3) then allows for connecting both
“Zn3” parts to assemble into the hexanuclear species. Note that
in both bis-salphen scaffolds there is a biphenyl spacer that
can have two distinct chiral conformations having either M or
P-helicity.10
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9766–9772 | 9769

View Online
[Zn2L(HOAc)]. This information is of high importance, since
these bis-Zn(salphen) scaffolds are excellent supramolecular host
systems for chirogenesis effects within the context of the deter-
mination of absolute configurations of various ditopic substrates;
these ditopic substrates bind to both Zn ions and competitive
binding of anionic ligands compromises the effectiveness of the
host systems. Thus the present results help to understand and
design new host systems that do not contain yet a competitive
guest that needs to be exchanged prior to the chirogenesis
process. The current focus is therefore on the design and iso-
lation of acid- and/or salt-free host systems and results will be
reported in due course.
Experimental
General comments
Scheme 3 Synthesis of HOAc-complex [Zn2L1(HOAc)] 5 and side-
product 6.
All reagents are commercially available and were used as
received. NMR spectra were recorded on a Bruker AV-400 or
AV-500 spectrometer and referenced to the residual deuterated
solvent signals. Elemental analysis was performed by the
Unidád de Análisis Elemental at the University of Santiago de
Compostela (Spain). Mass spectrometric analysis and X-ray dif-
fraction studies were performed by the Research Support Area at
the ICIQ. Both mono-imine reagents A, B and precursor ligand
L4-H4 were prepared according to previously published pro-
cedures.11,13 The aldehydes 3,3′-diformyl-2,2′-dihydroxy-1,1′-
biphenyl14 and (S)-2,2′-dihydroxy-[1,1′-binaphthalene]-3,3′-di-
aldehyde15 were prepared as reported.
this hypothesis, we combined 3,3′-diformyl-2,2′-dihydroxy-1,1′-
biphenyl (Scheme 3) with mono-imine A in the presence of a
slight deficit of Zn(OAc)2 using similar reaction conditions as
reported for [Zn3L1(OAc)2] 1 (Experimental section). An orange
1
solid was isolated that was analyzed by H NMR spectroscopy;
comparison with authentic data revealed the presence of two
species, viz. binuclear complex [Zn2L1(HOAc)] 5 and the
known mononuclear Zn(salphen) derivative 612 in a 1 : 1.3 ratio
(see ESI†). Recrystallization from warm MeOH afforded pure
[Zn2L1(HOAc)] 5 that was identified as a bis-Zn(salphen)
complex with one molecule of associated HOAc; this is sup-
ported by the relative integral ratio between the imine-H of the
binuclear host (4H) and the HOAc resonance at δ = 1.90 ppm
(3H). Furthermore, there is a clear difference in chemical shift
(δ = 1.90 ppm for [Zn2L1(HOAc)]) as opposed to the OAc frag-
ments in complex [Zn3L1(OAc)2] 1 (δ = 1.81 ppm, see ESI†).
Also, the 13C NMR analysis showed a significant difference
between trinuclear [Zn3L1(OAc)2] 1 (δ = 22.79 ppm) and binuc-
lear [Zn2L1(HOAc)] 5 (δ = 21.60 ppm) for the OAc fragments.
Therefore, the synthesis of 5 in conjunction with the data for the
previously reported structure (Fig. 1) thus supports that the reac-
tion stoichiometry is indeed a decisive parameter. The obser-
vation of low to moderate yields for multinuclear 1–3 and
dinuclear 5 (17–61%) is ascribed to side-product formation that
is caused by in situ equilibration of the mono-imines A and B
giving the fully condensed salphen ligands that, upon metalation,
can furnish mononuclear Zn(salphen)s such as 6.
Preparation of trinuclear complex [Zn3L1(OAc)2] 1. To a
solution of 3,3′-diformyl-2,2′-dihydroxy-1,1′-biphenyl (93 mg,
0.38 mmol) and the mono-imine reagent
A (252 mg,
0.77 mmol) in CHCl3 (20 mL) was added a solution of
Zn(OAc)2·2H2O (182 mg, 0.83 mmol) dissolved in MeOH–pyri-
dine (15 mL, 2 : 1 v/v). The dark orange solution was stirred for
18 h after which it was concentrated. Addition of CH3CN caused
precipitation of an orange solid that was collected by filtration
and dried (116 mg). A second fraction of the product could be
obtained by cooling the mother liquor to 6 °C (60 mg). Total
yield: 176 mg (0.179 mmol, 47% based on dialdehyde). Crystals
suitable for X-ray diffraction were obtained from CH3CN in the
presence of n-hexylamine. 1H NMR (400 MHz, [d6]DMSO):
δ = 9.00 (s, 2H; CHvN), 8.94 (s, 2H; CHvN), 7.99 (d, 3JH,H
=
4
7.1 Hz, JH,H = 1.8 Hz, 2H; ArH), 7.84–7.88 (m, 4H; ArH),
4
7.33–7.36 (m, 6H; ArH), 7.30 (d, JH,H = 2.5 Hz, 2H; ArH),
4
3
7.20 (d, JH,H = 2.6 Hz, 2H; ArH), 6.60 (t, JH,H = 7.5 Hz, 2H;
ArH), 1.81 (s, 6H; OAc), 1.46 (s, 18H; C(CH3)3), 1.27 (s, 18H;
C(CH3)3); 13C{1H} NMR (100 MHz, [d6]acetone): δ = 170.62,
163.62, 163.26, 141.23, 140.37, 140.10, 137.80, 134.95, 133.66,
130.82, 129.96, 128.76, 127.24, 127.07, 119.94, 118.66, 116.73,
116.65, 112.89, 36.15, 35.59, 31.80, 30.21, 22.79; MS
(MALDI+, dctb): m/z = 1168.4 (M)+, 982.3 (M − Zn(OAc)2)+
(calcd 982.3). Elemental analysis calcd (%) for C60H64N4O8Zn3·
3H2O: C 59.10, H 5.79, N 4.59; found: C 59.05, H 5.26, N 4.49.
Conclusions
In this contribution we have detailed the influence of the reaction
stoichiometry on the outcome of the metalation process for
various (chiral) bis-salphen scaffolds. In the case of an excess of
Zn reagent, incorporation of Zn(OAc)2 or Zn(OH)2 salts via
coordinative patterns that involve either bridging OAc or
OH anions is observed leading to structures with the formula
[Zn3L(OAc)2]2 or [Zn3L(OH)2]2 while the use of a slight deficit
of Zn reagent leads to the incorporation of HOAc as
previously observed giving structures generally described as
Preparation of chiral trinuclear complex [Zn3L2(OAc)2] 2.
(S)-2,2′-Dihydroxy-[1,1′-binaphthalene]-3,3′-dialdehyde (51 mg,
0.15 mmol) and monoimine reagent A (104 mg, 0.32 mmol)
were dissolved in DCM (5 mL). To this mixture was added a
9770 | Dalton Trans., 2012, 41, 9766–9772
This journal is © The Royal Society of Chemistry 2012

View Online
3
4
3
solution of Zn(OAc)2·2H2O (77 mg, 0.35 mmol) dissolved in
MeOH (2.5 mL) and a few drops of NEt3. The solution was left
stirring for 6 h and then cooled to −30 °C to induce precipitation
of the product. The desired compound was isolated by filtration
and dried in vacuo to yield an orange solid (81 mg, 43% based
(t, JH,H = 7.8, JH,H = 1.8 Hz, 8H; ArH), 6.49 (dd, JH,H = 8.2,
4JH,H = 1.1 Hz, 8H; ArH), 6.54 (dd, 3JH,H = 7.2, 4JH,H = 2.0 Hz,
3
4
8H; ArH), 6.62 (dd, JH,H = 8.59, JH,H = 1.24 Hz, 8H; ArH),
6.70 (dd, 3JH,H = 8.5, 4JH,H = 1.2 Hz, 8H; ArH), 6.81 (t, JH,H
=
=
3
7.7, 4JH,H = 1.4 Hz, 8H; ArH), 6.85 (dd, 3JH,H = 8.3 Hz, 4JH,H
1
3
4
on dialdehyde). H NMR (400 MHz, [d6]acetone): δ = 9.40 (s,
1.2 Hz, 8H; ArH), 6.92 (dd, JH,H = 8.4, JH,H = 1.7 Hz, 8H;
ArH), 6.98 (dd, JH,H = 7.7, JH,H = 1.3 Hz, 8H; ArH), 7.04 (m,
32H; ArH), 7.10 (dd, JH,H = 8.2, JH,H = 1.8 Hz, 16H; ArH),
7.21 (t, JH,H = 7.7, JH,H = 1.2 Hz, 16H; ArH), 7.32 (m, 40H;
ArH), 7.43 (m, 16H; ArH), 7.50 (t, JH,H = 7.9, JH,H = 1.4 Hz,
16H; ArH), 7.62 (dd, JH,H = 8.0, JH,H = 2.0 Hz, 8H; ArH),
7.70 (dd, JH,H = 8.3, JH,H = 1.3 Hz, 8H; ArH), 8.08 (s, 8H;
CHvN), 8.79 (s, 8H; CHvN); 13C{1H} NMR (125 MHz,
CD2Cl2, mixture of four diastereoisomers in a 1 : 1 : 1 : 1 ratio):
δ = 112.66, 115.61, 116.20, 116.31, 116.83, 117.21, 120.31,
120.46, 122.13, 122.95, 123.17, 123.69, 123.84, 124.15, 125.56,
125.85, 126.21, 126.71, 127.96, 128.04, 128.20, 128.27, 128.34,
128.39, 132.18, 132.54, 133.54, 133.86, 134.67, 134.73, 135.12,
135.81, 137.22, 137.30, 137.43, 139.31, 140.23, 140.51, 140.81,
141.20, 162.80, 164.19, 165.05, 166.40, 167.97, 171.32, 173.40,
174.63; MS (MALDI+, dctb): m/z = 1983.3 (M − 2OH)+ (calcd
1983.1). Elemental analysis calcd (%) for C104H72N8O12Zn6·
7H2O: C 58.26, H 4.04, N 5.23; found: C 58.37, H 3.74, N 5.08.
3
4
2H; CHvN), 8.86 (s, 2H; CHvN), 8.55 (s, 2H; ArH), 8.07 (d,
3
3
4
3JH,H = 8.1 Hz, 2H; ArH), 7.95 (d, JH,H = 7.8 Hz, 2H; ArH),
3
3
3
4
7.86 (d, JH,H = 7.1 Hz, 2H; ArH), 7.50 (t, JH,H = 8.7 Hz, 2H;
ArH), 7.45 (t, 3JH,H = 8.5 Hz, 2H; ArH), 7.34 (t, 3JH,H = 8.9 Hz,
2H; ArH), 7.22 (t, overlapping peak J not resolved, 2H; ArH),
3
4
3
4
4
4
3
4
7.21 (d, JH,H = 2.9 Hz, 2H; ArH), 7.09 (d, JH,H = 2.5 Hz, 2H;
3
ArH), 7.00 (d, JH,H = 8.7 Hz, 2H; ArH), 1.53 (s, 6H; OAc),
1.23 (s, 18H; C(CH3)3), 0.71 (s, 18H; C(CH3)3); 13C{1H} NMR
(100 MHz, [d6]acetone): δ = 171.08, 170.45, 165.27, 163.12,
158.36, 152.22, 141.57, 140.87, 140.05, 139.07, 137.42, 134.13,
129.38, 128.99, 128.92, 128.73, 128.20, 127.18, 126.07, 124.68,
123.24, 122.97, 118.51, 117.70, 115.92, 34.56, 33.39, 30.79;
MS (MALDI+, dctb): m/z = 1264.3 [M]+ (calcd 1264.3), 1205.3
[M − (OAc)]+ (calcd 1205.3), 1080.3 [M − Zn(OAc)2]+ (calcd
1080.3). Elemental analysis calcd (%) for C66H66N4O6Zn2·
Zn(OAc)2·1.5H2O: C 63.19, H 5.54, N 4.33; found: C 63.24,
H 5.51, N 4.37.
Preparation of dinuclear complex [Zn2L1(HOAc)] 5. To a
solution of 3,3′-diformyl-2,2′-dihydroxy-1,1′-biphenyl (81.9 mg,
0.338 mmol) and the mono-imine reagent A (226.1 mg,
0.697 mmol) in CHCl3 (20 mL) was added a solution of
Zn(OAc)2·2H2O (141.1 mg, 0.643 mmol) dissolved in MeOH–
pyridine (10 mL : 1 mL). The orange solution was stirred for 4 h
after which it was concentrated. Addition of CH3CN caused pre-
cipitation of an orange solid that was collected by filtration and
dried (178.7 mg). This product was shown to be a mixture of 5
Preparation of chiral trinuclear complex [Zn3L3(OAc)2] 3.
(S)-2,2′-Dihydroxy-[1,1′-binaphthalene]-3,3′-dialdehyde (49 mg,
0.14 mmol) and mono-imine reagent B (79 mg, 0.29 mmol)
were dissolved in DCM (5 mL). To this mixture was added a
solution of Zn(OAc)2·2H2O (82 mg, 0.37 mmol) dissolved in
MeOH (2.5 mL) and a few drops of NEt3. The solution was left
stirring for 6 h and then cooled to −30 °C to induce precipitation
of the product. The desired compound was isolated by filtration
and dried in vacuo to yield an orange solid (98 mg, 61% based
1
1
and 6 in a 1 : 1.3 ratio by H NMR signal integration. Recrystal-
on dialdehyde). H NMR (500 MHz, [d6]DMSO): δ = 9.28 (s,
2H; CHvN), 8.73 (s, 2H; CHvN), 8.17 (s, 2H; ArH), 7.92 (m,
lization of this mixture from warm MeOH afforded pure 5, while
subsequent fractions contained impure material. Yield: 57.1 mg
(0.0548 mmol, 17% based on the Zn reagent). 1H NMR
(400 MHz, [d6]DMSO): δ = 8.99 (s, 2H; CHvN), 8.94 (s, 2H;
2H; ArH), 7.74 (m, 4H; ArH), 7.40 (m, 4H; ArH), 7.09 (d,
4
3JH,H = 8.1 Hz, JH,H = 1.6 Hz; ArH), 6.97 (m, 8H; ArH), 6.26
3
(t, JH,H = 8.5 Hz; ArH), 1.81 (s, 6H; OAc), 0.91 (s, 18H;
3
4
C(CH3)3); 13C{1H} NMR (125 MHz, [d6]DMSO): δ = 172.53,
165.47, 164.28, 163.94, 141.95, 141.17, 139.82, 138.33, 137.37,
134.66, 130.56, 129.36, 128.28, 127.40, 127.19, 125.21, 124.86,
123.90, 123.81, 119.85, 119.56, 117.14, 116.93, 112.22, 100.03,
34.82, 29.77, 22.79; MS (MALDI+, dctb): m/z = 1152.3 [M]+
(calcd 1152.2), 1093.3 [M − (OAc)]+ (calcd 1093.2), 968.3
[M − Zn(OAc)2]+ (calcd 968.2). Elemental analysis calcd (%)
for C58H50N4O6Zn2·Zn(OAc)2·H2O: C 61.53, H 4.65, N 4.78;
found: C 61.33, H 4.91, N 4.74.
CHvN), 8.00 (d, JH,H = 7.1 Hz, JH,H = 1.7 Hz, 2H; ArH),
7.84–7.88 (m, 4H; ArH), 7.34–7.37 (m, 6H; ArH), 7.30 (d,
4
4JH,H = 2.4 Hz, 2H; ArH), 7.21 (d, JH,H = 2.4 Hz, 2H; ArH),
3
6.61 (t, JH,H = 7.5 Hz, 2H; ArH), 1.90 (s, 3H; OAc), 1.47 (s,
18H; C(CH3)3), 1.27 (s, 18H; C(CH3)3); 13C{1H} NMR
(100 MHz, [d6]acetone): δ = 170.63, 163.64, 163.28, 141.25,
140.39, 140.12, 137.82, 134.98, 133.68, 130.84, 129.98, 128.80,
127.26, 127.09, 119.96, 118.68, 116.76, 116.69, 112.92, 35.62,
33.99, 31.82, 30.22, 21.60; MS (MALDI+, dctb): m/z = 982.4
(M − HOAc)+ (calcd 982.3). Elemental analysis calcd (%) for
C58H62N4O6Zn2·4H2O: C 62.54, H 6.33, N 5.03; found:
C 62.37, H 6.33, N 5.20.
Preparation of chiral hexanuclear complex [Zn3L4(OH)2]2
4. To a suspension of ligand C (200 mg, 0.26 mmol) in an-
hydrous hexane (5 mL) was slowly added ZnEt2 (0.77 mL 1 M
in hexanes, 0.77 mmol) under a nitrogen atmosphere. The result-
ing mixture was stirred for 18 h. The precipitate that had formed
was then filtered off to give an orange solid. Yield: 235 mg
(0.12 mmol of Zn6, 92% based on the ligand precursor). Crystals
suitable for X-ray diffraction studies were obtained from DCM.
1H NMR (500 MHz, CD2Cl2, mixture of four diastereoisomers
1 : 1 : 1 : 1 ratio): δ = −0.64 (s, 8H; OH), 0.12 (s, 8H; OH),
Crystallographic studies
General: The measured crystals were stable under atmospheric
conditions; nevertheless they were treated under inert conditions
immersed in perfluoropoly-ether as a protecting oil for manipu-
lation. Data collection: Measurements were made on a Bruker-
Nonius diffractometer equipped with an APPEX 2 4K CCD area
detector, an FR591 rotating anode with MoKα radiation, Montel
3
6.20 (m, 16H; ArH), 6.30 (t, JH,H = 7.5 Hz, 8H; ArH), 6.45
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9766–9772 | 9771

View Online
2 (a) R. M. Haak, S. J. Wezenberg and A. W. Kleij, Chem. Commun.,
2010, 46, 2713; (b) A. Decortes, A. M. Castilla and A. W. Kleij, Angew.
Chem., Int. Ed., 2010, 49, 9822; (c) E. N. Jacobsen, Acc. Chem. Res.,
2000, 33, 421; (d) P. G. Cozzi, Chem. Soc. Rev., 2004, 33, 410;
(e) M. Shibasaki, H. Sasai and T. Arai, Angew. Chem., Int. Ed. Engl.,
1997, 36, 1236.
3 (a) G. Consiglio, S. Failla, P. Finocchiaro, I. P. Oliveri, R. Purrello and
S. Di Bella, Inorg. Chem., 2010, 49, 5134; (b) G. Consiglio, S. Failla,
I. P. Oliveri, R. Purrello and S. Di Bella, Dalton Trans., 2009, 10426;
(c) S. J. Wezenberg, E. C. Escudero-Adán, J. Benet-Buchholz and
A. W. Kleij, Org. Lett., 2008, 10, 3311; (d) A. C. W. Leung and
M. J. J. MacLachlan, J. Mater. Chem., 2007, 17, 1923; (e) C. T. L. Ma
and M. J. MacLachlan, Angew. Chem., Int. Ed., 2005, 44, 4178;
(f) R. M. Haak, A. Decortes, E. C. Escudero-Adán, M. Martínez
Belmonte, E. Martin, J. Benet-Buchholz and A. W. Kleij, Inorg. Chem.,
2011, 50, 7934; (g) M. Ishida, J.-Y. Shin, J. M. Lim, B. S. Lee,
M.-C. Yoon, T. Koide, J. L. Sessler, A. Osuka and D. Kim, J. Am. Chem.
Soc., 2011, 133, 15533.
4 (a) M. MacLachlan, Pure Appl. Chem., 2006, 78, 873;
(b) S. J. Wezenberg and A. W. Kleij, Angew. Chem., Int. Ed., 2008, 47,
2354; (c) S. Guieu, A. K. Crane and M. J. MacLachlan, Chem. Commun.,
2011, 47, 1169; (d) S. Akine, T. Taniguchi and T. Nabeshima, J. Am.
Chem. Soc., 2006, 128, 15765; (e) J. A. A. W. Elemans, S. J. Wezenberg,
M. J. J. Coenen, E. C. Escudero-Adán, J. Benet-Buchholz, D. den Boer,
S. Speller, A. W. Kleij and S. De Feyter, Chem. Commun., 2010, 46,
2548; (f) J. L. Sessler, S. Camiolo and P. A. Gale, Coord. Chem. Rev.,
2003, 240, 17.
5 (a) J. L. Sessler, E. Tomat and V. M. Lynch, J. Am. Chem. Soc.,
2006, 128, 4184; (b) J. L. Sessler, E. Katayev, G. D. Pantos,
P. Scherbakov, M. D. Reshetova, V. N. Khrustalev, V. M. Lynch and
Y. A. Ustynyuk, J. Am. Chem. Soc., 2005, 127, 11442; (c) A. Dalla
Cort, P. De Bernardin, G. Forte and M. F. Yafteh, Chem. Soc. Rev.,
2010, 39, 3863.
mirrors and a Kryoflex low temperature device (T = −173 °C).
Full-sphere data collection was used with ω and φ scans.
Programs used: Data collection Apex2 V2011.3 (Bruker-Nonius
2008), data reduction Saint + Version 7.60A (Bruker AXS 2008)
and absorption correction SADABS V. 2008-1 (2008). Structure
solution: SHELXTL Version 6.10 (Sheldrick, 2000)16 was used.
Structure refinement: SHELXTL-97-UNIX VERSION.
Crystal data for [Zn3L1(OAc)2] 1·CH3CN: C80H112N8O8Zn3,
ˉ
Mr = 1509.89, triclinic, P1, a = 15.2147(15) Å, b = 17.6284(18) Å,
c = 17.8255(16) Å, α = 112.366(3)°, β = 113.184(2)°, γ =
90.041(3)°, V = 4000.1(7) Å3, Z = 2, ρ = 1.254 mg M−3, μ =
0.949 mm−1, λ = 0.71073 Å, T = 100(2) K, F(000) = 1604,
crystal size = 0.15 × 0.06 × 0.03 mm, θ (min) = 1.27°, θ (max)
= 30.68°, 15 867 reflections collected, 15 417 reflections unique
(Rint = 0.0139), GoF = 1.045, R1 = 0.0723 and wR2 = 0.1959
[I > 2σ(I)], R1 = 0.1338 and wR2 = 0.2403 (all indices),
min/max residual density = −0.868/1.168 [e Å−3]. Note the
completeness to θ (30.68°) is only 62.1%; this is due to the long
time required for collecting the data which caused ice formation,
and the measurement had to be interrupted. Nonetheless, the
connectivity pattern is clearly established. Data collection with a
long exposure time has been made to reach the maximal data
resolution in order to model the disorder. Higher resolution data
allow us to have a good data-to-parameter ratio (15 : 1 for the
current structure). The structure has been deposited at the CCDC
with reference number 871926.
Crystal data for [Zn3L4(OH)2]2 4·5.5(ClCH2CH2Cl)·2.5-
(CH2Cl2): C117.5H99N8O12Cl16Zn6, Mr = 2774.46, monoclinic,
C2/c, a = 26.743(2) Å, b = 17.5255(15) Å, c = 24.944(2) Å, α =
90°, β = 104.816(3)°, γ = 90°, V = 11302.2(17) Å3, Z = 4, ρ =
1.631 mg M−3, μ = 1.698 mm−1, λ = 0.71073 Å, T = 100(2) K,
F(000) = 5632, crystal size = 0.30 × 0.20 × 0.10 mm, θ (min) =
1.74°, θ (max) = 31.87°, 306 121 reflections collected, 19 311
reflections unique (Rint = 0.0500), GoF = 1.037, R1 = 0.0533
and wR2 = 0.1423 [I > 2σ(I)], R1 = 0.0720 and wR2 = 0.1578
(all indices), min/max residual density = −1.626/1.106 [e Å−3].
Completeness to θ (31.87°) = 99.6%. The structure has been
deposited at the CCDC with reference number 871927.
6 See for instance: R. G. Konsler, J. Karl and E. N. Jacobsen, J. Am. Chem.
Soc., 1998, 120, 10780.
7 See for a recent review: A. W. Kleij, Chem.–Eur. J., 2008, 14,
10520.
8 (a) J. K.-H. Hui and M. J. MacLachlan, Chem. Commun., 2006, 2480;
(b) P. D. Frischmann, J. Jiang, J. K.-H. Hui, J. J. Grzybowski and
M. J. MacLachlan, Org. Lett., 2008, 10, 1255; (c) G. Salassa,
A. M. Castilla and A. W. Kleij, Dalton Trans., 2011, 40, 5236;
(d) J. Barreira-Fontecha, R. Kulmaczewski, X. Ma and V. McKee, Dalton
Trans., 2011, 40, 12040; (e) S. Akine, S. Sunaga and T. Nabeshima,
Chem.–Eur. J., 2011, 17, 6853; (f) G. Givaja, A. J. Blake, C. Wilson,
M. Schröder and J. B. Love, Chem. Commun., 2005, 4423; (g) R. Sanyal,
S. A. Cameron and S. Brooker, Dalton Trans., 2011, 40, 12277;
(h) S. Akine, S. Sunaga, T. Taniguchi, H. Miyazaki and T. Nabeshima,
Inorg. Chem., 2007, 46, 2959.
9 A. M. Castilla, S. Curreli, M. Martínez Belmonte, E. C. Escudero-Adán,
J. Benet-Buchholz and A. W. Kleij, Org. Lett., 2009, 11, 5218.
10 S. J. Wezenberg, G. Salassa, E. C. Escudero-Adán, J. Benet-Buchholz
and A. W. Kleij, Angew. Chem., Int. Ed., 2011, 50, 713.
Acknowledgements
11 M. V. Escárcega-Bobadilla, G. Salassa, M. Martínez Belmonte,
E. C. Escudero-Adán and A. W. Kleij, Chem.–Eur. J., 2012, 18, 6805.
12 A. W. Kleij, D. M. Tooke, M. Kuil, M. Lutz, A. L. Spek and
J. N. H. Reek, Chem.–Eur. J., 2005, 11, 4743.
13 For the synthesis of mono-imine A see: M.-A. Muñoz-Hernández,
T. S. Keizer, S. Parkin, B. Patrick and D. A. Atwood, Organo-
metallics, 2000, 19, 4416; for the synthesis of mono-imine B see
ref. 10.
This work was supported by ICIQ, ICREA and the Spanish Min-
isterio de Economía y Competitividad (MINECO) through
project CTQ2011-27385. We thank Dr Noemí Cabello and Sofia
Arnal for the mass spectrometric studies.
14 H.-C. Zhang, W.-S. Huang and L. Pu, J. Org. Chem., 2001, 66, 481.
15 Y. N. Belokon, D. Chusov, D. A. Borkina, L. V. Yashkina,
A. V. Dmitriev, D. Katayev and M. North, Tetrahedron: Asymmetry,
2006, 17, 2328.
16 G. M. Sheldrick, SHELXTL Crystallographic System, Version 6.10,
Bruker AXS, Inc., Madison (Wisconsin), 2000.
Notes and references
1 For some recent examples: (a) P. W. Roesky, A. Bhunia, Y. Lan,
A. K. Powell and V. Kureti, Chem. Commun., 2011, 47, 2035;
(b) H. L. C. Feltham, R. Clérac, A. K. Powell and S. Brooker, Inorg.
Chem., 2011, 50, 4232.
9772 | Dalton Trans., 2012, 41, 9766–9772
This journal is © The Royal Society of Chemistry 2012