Shape-persistent octanuclear zinc salen clusters: Synthesis, characterization, and catalysis

Article abstract of DOI:10.1021/ic201064d

We describe a selective and template-controlled synthesis of a series of Zn₈ metal complexes based on a bis-nucleating salen ligand scaffold. Our results, a combination of X-ray analysis and solution studies, show that discrete, shape-persistent metal clusters can be prepared in high yield. Their activity in organic carbonate catalysis is a function of the metal-connecting fragment present in the exterior of the cluster complex. The high stability of the clusters has been confirmed by ¹H, ¹³C (DEPTQ) and DOSY NMR, gel permeation chromatography, high-performance liquid chromatography, and mass spectrometry.

Full text of DOI:10.1021/ic201064d

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pubs.acs.org/IC
Shape-Persistent Octanuclear Zinc Salen Clusters: Synthesis,
Characterization, and Catalysis
Robert M. Haak,† Antonello Decortes,† Eduardo C. Escudero-Adꢀan,† Marta Martínez Belmonte,†
Eddy Martin,† Jordi Benet-Buchholz,† and Arjan W. Kleij*,†,‡
†Institute of Chemical Research of Catalonia (ICIQ), Avenida Països Catalans 16, 43007 Tarragona, Spain
‡Catalan Institute for Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain
S Supporting Information
b
ABSTRACT: We describe a selective and template-con-
trolled synthesis of a series of Zn8 metal complexes based on
a bis-nucleating salen ligand scaffold. Our results, a combi-
nation of X-ray analysis and solution studies, show that
discrete, shape-persistent metal clusters can be prepared in
high yield. Their activity in organic carbonate catalysis is a
function of the metal-connecting fragment present in the
exterior of the cluster complex. The high stability of the
1
clusters has been confirmed by H, 13C (DEPTQ) and
DOSY NMR, gel permeation chromatography, high-perfor-
mance liquid chromatography, and mass spectrometry.
he selective synthesis of defined cluster compounds remains
an attractive challenge because these may offer materials
T
with interesting photochemical and magnetic properties, and self-
assembly features.1 In this respect, Schiff base ligands such as
salens have proven to be excellent directional ligands, in parti-
cular in those cases where additional O-donor atoms have been
introduced in the backbone of the ligand scaffold.2 Such systems
are best designated as bis-nucleating because the usual central
N2O2 coordination pocket is flanked by an outer shell of four O
atoms, forming a O4 cavity into which a wide variety of cations
can be assembled.3 Selective introduction of metal ions in the
N2O2 and O4 pockets can be achieved to furnish heterometallic
complexes that can be of great use in (asymmetric) catalysis.4
Shibasaki et al. have reported on such heterodinuclear Schiff base
complexes that proved to be powerful catalysts in organic
synthesis.5 The metal ion in the outer “O2ÀO2” coordination
shell may help to shape the coordination geometry around the
metal ion in the N2O2 salen pocket, thereby offering a way to
fine-tune important catalytic objectives such as enantio- and
diastereoselectivity. Despite the effectiveness of such bimetallic
derivatives in asymmetric catalysis, in most cases there is little
information available on the structural features that are related
with the actual catalysts, and higher-order cluster compounds
have been proposed and observed mainly on the basis of mass
spectrometric evidence.6 Bermejo et al. reported on the forma-
tion of octanuclear Zn8 clusters based on bis-nucleating salen
scaffolds (for an example, see Figure 1, X = C).7 In this case,
flexible spacers between the salicylidene units were probed,
although chiral diamines were not investigated in the assembly
process. At a later stage, Nabeshima et al. utilized a structurally
related salamo (or bis-oxime) type ligand (Figure 1, X = O),
Figure 1. Bis-nucleating salen/salamo ligands used by Bermejo and
Nabeshima and ligands (L1ÀL7) used in this work.
which resulted, upon metalation with zinc(II), in a similar Zn8
cluster derivative.8 Despite the fact that both types of cluster
compounds were easily formed, the scope of these cluster
complexes has been surprisingly limited to only a few examples
and chiral analogues have thus far not been reported. Further-
more, catalytic application of these clustered complexes was
neither intended nor proposed, which may have been a result of
the observed fragmentation under mass spectral conditions.
Here, we report on a series of new octanuclear Zn8 cluster com-
plexes that have been conveniently assembled in a single step
from readily available precursors. Beside nonchiral diamines, we
have also investigated the incorporation of various chiral dia-
mines to give chiral cluster compounds. Their stability in solution
has been studied in detail, and these investigations have shown
these new cluster compounds to be shape-persistent and having
molecular structures similar to those found in the solid state.
Interestingly, the reactivity of these Zn8 cluster compounds can
be modified by the introduction of appropriate bridging groups,
as illustrated by their application in CO2 fixation catalysis.9
Received: May 19, 2011
Published: August 01, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
COMMUNICATION
μ2-phenoxo bridges between two Zn ions. Interestingly, the
space available for axial coordination of external ligands to the
Zn ions ligated by the N2O2 sections of these binucleating
systems seems to be limited because the interatomic distance
from the coordinated H2O ligand and the nearest O atom of an
adjacent ligand is only ∼2.5 Å. The molecular radius of tetra-
meric L1Zn2 (about 10À10.5 Å) corresponds well with that
derived from the DOSY experiment, further supporting the
presence of the octanuclear structure in solution.
Because only a few of these Zn8 cluster compounds have been
reported and the selective formation and stability have been the
subject of some debate, we then investigated the influence of the
flexibility of the diamine reagent on the outcome of the assembly
process (L1ÀL7 in Figure 1). Using a similar synthetic protocol
and in those cases where ligands L2ÀL4 were utilized, octanuclear
cluster complexes could be conveniently isolated in moderate-to-
good yields (Scheme S1). All compounds displayed a single peak in
the mass spectra and single NMR resonance patterns (see the SI),
indicating a high molecular stability. The X-ray structures for
[L4aZn2]4 and [L4bZn2]4 were also solved (see the SI), and
crystals were grown from DMF or DMF/MeOH; surprisingly, the
exterior Zn ions located in the N2O2 coordination pockets are
axially coordinated by H2O (for [L4aZn2]4) or MeOH (for
[L4bZn2]4), a result that points to steric infringement around these
metal centers, preventing coordination of the larger DMF. When a
rather flexible diamine precursor such as 1,8-diaminooctane was
explored, an insoluble material resulted, which may be due to the
formation of a polymeric structure rather than a discrete assembly,
as observed with L1ÀL4. Also, in the case of L5, a dimeric rather
than a tetrameric assembly was supported by mass spectrometry
(see the SI). Whereas L1 in the presence of excess Zn(OAc)2 yields
exclusively octanuclear complexes, in the case of L6 and L7
(Figure 1), the selective formation of monozinc complexes was
observed (see the SI and Scheme 1; the complex based on L7 was
contaminated with an aldehyde reagent); this clearly demonstrates
the requirement for close proximity of the O-atom donors to
mediate the formation of larger discrete assemblies.
Figure 2. Crystal structure for [L1Zn2]4 showing an octanuclear
complex. Cocrystallized solvent molecules and H atoms are omitted
for clarity, and only a partial labeling scheme is illustrated.
As a starting point for our studies, we chose chiral ligand L1
and divalent zinc as the metal ion because it allows for various
coordination geometries and previous results with comparable
dinucleating ligands gave access to high-nuclear zinc cluster
complexes.7,8 While the coordination geometry inside the N2O2
salen pocket is relatively rigid, in the outer region of L1ÀL5, it is
obvious that such a multidentate ligation mode is restricted and
the assembly of multiple ions is required in this region. Upon
combination of L1 with 2 equiv of Zn(OAc)2 2H2O in refluxing
3
ethanol and isolation of the precipitate (97% yield) upon cooling,
a bright-yellow solid was obtained. This solid was analyzed by 1H
NMR [dimethylformamide (DMF)-d7] and showed the presence
of four diagnostic doublets for the bridgehead protons, indicating
the presence of a higher-order species. The matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) mass spec-
trum showed only one peak at m/z 2316.1 consistent with
molecular formula [L1Zn2]4, i.e., a tetrameric, octanuclear species
(see the Supporting Information, SI). The presence of multiple
zinc metal centers is particularly useful because they provide a true
molecular signature (isotopic pattern) and allow for quick identi-
fication of higher-order, clustered materials. In order to get more
insight in the solution structure, gel permeation chromatography
(GPC; THF = tetrahydrofuran), high-performance liquid chro-
matography (HPLC; THF), and diffusion-ordered spectroscopy
(DOSY; 1H NMR) analyses (see the SI) were additionally
performed. The GPC/HPLC trace showed only one peak in line
with a single species under these polar conditions. The DOSY
NMR was performed in both THF-d8 and DMF-d7, with both
cases showing similar results; the molecular radius (R = 10.5 Å)
was estimated using the StokesÀEinstein equation, which is
significantly beyond the dimensions of the monomeric unit
L1Zn2. Fortunately, crystals were obtained from acetone/
CH3CN, and the molecular structure is depicted in Figure 2.10,11
The molecular structure shows the assembly of four L1Zn2
units with the Zn ions in the N2O2 pockets having an axial
H2O ligand. Each of the “inner” Zn ions are surrounded by five
O-atom donors, of which from each N2O4 salen unit three form
The results from the crystallographic analyses prompted us to
investigate the catalytic reactivity of these clusters because the
H2O ligands should be fairly easy to displace by suitable sub-
strates and thus act as Lewis acid activators. As a model reaction,
we chose the cycloaddition reaction of carbon dioxide to terminal
epoxides to afford cyclic carbonates,9b because recent literature
has demonstrated that mononuclear Zn(salen)s are excellent
catalyst systems for this transformation under relatively mild
conditions.10 As a benchmark substrate, we selected epoxyhex-
ane, and NBu4I was utilized as a cocatalyst at 45 °C and pCO
=
2
10 bar. The solubility of the octanuclear cluster complexes
(except for [L1Zn2]4) in CH2Cl2 and methyl ethyl ketone
(MEK) was limited, and THF therefore had to be used instead
to compare the cluster complexes in the catalytic studies
(Table 1, entries 1À5). Only in the case of [L1Zn2]4 was a
significant amount of the cyclic carbonate observed (21% yield,
entry 5), whereas the use of all of the other Zn8 clusters showed
virtually no product formation (1À3%). This contrasts our
previous results with various mononuclear Zn(salen) catalysts
in this reaction because in these studies the salphen [salphen = N,
ꢀ
N-bis(salicylidene)imine-1,2-diaminobenzene] systems were
clearly the most active mediators in cyclic carbonate synthesis.
However, these salphen systems also show a higher affinity for
axial binding of O-donor ligands, which may effectively compete
with substrate binding. Therefore, it seems that the overall
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Inorganic Chemistry
COMMUNICATION
Table 1. Catalytic Performance of the Cluster Compounds in
the Cycloaddition of CO2 to Epoxidesa
’ ASSOCIATED CONTENT
S
Supporting Information. Full details on the synthesis
b
and characterization of all new compounds and X-ray crystal-
lographic data in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: akleij@iciq.es.
entry
catalyst
solvent
yieldb
1
(L2Zn2)4
(L3Zn2)4
(L4aZn2)4
(L4bZn2)4
(L1Zn2)4
L6Zn
THF
THF
THF
THF
THF
THF
DCMc
DCM
MEKc
MEK
MEK
MEK
3
1
2
’ ACKNOWLEDGMENT
3
1
This work was supported by ICIQ, ICREA, the Spanish
Ministry of Science and Innovation (Grant CTQ 2008-02050),
and Consolider Ingenio 2010 (Grant CSD 2006-0003). R.M.H.
thanks the MICINN for a TQ grant (PTQ-09-01-00044).
4
1
5
21
41
1
6
7
(L4bZn2)4
(L1Zn2)4
(L4bZn2)4
(L1Zn2)4
(L1Zn2)4
(L1Zn2)4
8
2
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9
22
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10
11d
12e
a Reactions carried out with epoxyhexane as the substrate (unless
stated otherwise) at 45 °C, pCO = 10 bar, 2.5 mol % catalyst
2
(i.e., reactive Zn ions) and 2.5 mol % cocatalyst (NBu4I), and
reaction time = 18 h. b Yield of carbonate product determined by
1H NMR (DMSO-d6) using mesitylene as the internal standard. c The
catalyst was only partially soluble. d The substrate was benzyl glycidyl ether.
e The substrate was propylene oxide; reaction time = 68 h.
reactivity of the outer Zn ions is determined by the axial substrate
binding ability, steric factors, and flexibility of the bridging
fragment in the salen ligand. Compared with the mononuclear
systems L6Zn (entry 6, 41% yield), the complex [L1Zn2]4 shows
a reduced overall activity.
A higher yield of carbonate product was achieved in MEK,
amounting to 35% in the case of [L1Zn2]4 and 22% with
[L4bZn2]4 (entries 9 and 10) despite the fact that the latter
catalyst is only partially soluble. Then we examined some other
substrates in the cycloaddition reaction using catalyst system
[L1Zn2]4. The benzyl glycidyl ether was converted to its
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conditions, while propylene carbonate was obtained in high yield
(87%, entry 12) after a 68 h reaction time.
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In summary, we have presented a new series of (chiral)
octanuclear Zn8 cluster complexes based on readily available
dinucleating ligands L1ÀL4 and the first example of a catalytic
application of these multinuclear systems. The analytical data
support that these octanuclear systems are stable in solution and
retain a shape-persistent, spherical structure. The outer-posi-
tioned Zn ions are axially coordinated by a H2O ligand, which can
be displaced by epoxidic substrates. These clusters were tested as
catalysts for the cycloaddition of carbon dioxide to these oxiranes
to give cyclic carbonates: the nature of the bridging fragment
gives a tool to optimize the reactivity of these cluster derivatives
in catalysis with [L1Zn2]4, surprisingly being the most active in
this series. A current focus is the use of these metal cluster
materials in other types of catalysis, while taking advantage of
their nanoscale dimensions useful in separation from product
and/or substrate flows.11
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