Topologically diverse shape-persistent bis-(Zn-salphen) catalysts: efficient cyclic carbonate formation under mild conditions

Article abstract of DOI:10.1039/c5cc08794e

By adopting a shape-persistent bimetallic design approach, high initial molecular turnover frequencies (up to 14 800 h^-1) for coupling of CO₂ with epoxides in conjunction with ⁿBu₄NI, plus excellent yields under mild conditions (1 bar of CO₂, 45°C) have been achieved for catalysts containing cofacial Zn-salphen units.

Full text of DOI:10.1039/c5cc08794e

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Topologically diverse shape-persistent bis-(Zn-salphen) catalysts:
efficient cyclic carbonate formation under mild conditions†
Received 00th January 20xx,
Accepted 00th January 20xx
Shixiong He, Fuli Wang, Wah-Leung Tong, Shek-Man Yiu and Michael C. W. Chan*
DOI: 10.1039/x0xx00000x
www.rsc.org/
By adopting a shape-persistent bimetallic design approach, high systems tethered by an inert, rigid “anchor” component, which
initial molecular turnover frequencies (up to 14800 h–1) for incorporate the following design features: (1) any potential M–
coupling of CO2 with epoxides in conjunction with nBu4NI, plus
M
interaction would be precluded, and coordinative
excellent yields under mild conditions (1 bar of CO2, 45 °C), have unsaturation conserved, by the rigid anchor as well as the
been achieved for catalysts containing cofacial Zn-salphen units.
introduction of bulky substituents, and (2) variations in
molecular conformations are effectively limited to axial
rotation of the M-salphen units, yet such rotations can crucially
enable subtle adjustments in intermetallic distances and
geometry. Our objective was to modify and control the relative
spatial arrangement and orientation (“topology”) of the cofacial
M-salphen units, by changing the nature and dimensions of the
metal-organic group (to confer differing degrees of axial
rotation) and anchor component (xanthene or dibenzofuran), so
that the capacity of the resultant scaffolds for binding and
activation of small molecules can be probed and adapted. It is
proposed that a tunable, convergent reaction space between the
metal centers, capable of engendering enhanced reactivity and
potential cooperativity, can be created. Herein, a series of (Zn-
salphen)2 frameworks (Fig. 1) have been prepared and their
ability to catalyze cyclic carbonate formation from CO2 and
epoxide have been examined. High efficiencies and turnover
frequencies are achieved under mild conditions, and
comparisons are made with the mononuclear control M1, which
is a benchmark example of active catalysts.7
Carbon dioxide is a particularly attractive C1 feedstock because
it is sustainable, economical and non-toxic, and its
transformation into useful chemicals and energy can reduce
waste.1 Although the thermodynamics and kinetics of reactivity
are unfavorable, the development of complexes and catalysts
for CO2 activation and utilization has proliferated,2 and at the
forefront of this has been the metal-catalyzed coupling of CO2
and epoxide to give organic (cyclic3 and/or poly-4) carbonates.
Mononuclear Zn(II) complexes supported by tetradentate
R-1,2-bis(salicylideneimine) auxiliaries (R
binaphthyl,5
phenylene (salphen)6,7), as well as Al(III) relatives bearing
amino-tris(phenolate)8 (plus binuclear [(Al-salen)2( -O)]9), can
N,N’-
=
μ
function as highly active catalysts for the synthesis of cyclic
carbonates. With regards to bimetallic Zn(II) catalysts, the
employment of
β
-diiminate (formally monomeric),10
pyrrolidinyl-methoxide,11 and macrocyclic anilido-aldimine12
and tetraamine-bis(phenolate)13 ligands resulted in the
production of polycarbonates, while tri-14 and tetranuclear
derivatives such as Zn4(OCOCF3)6(μ
-O)15 displayed enhanced
The modular synthetic method employed in this work
connects two Zn-bound salphen (or salen) moieties, which are
robust and easily derivatized, with a bridging anchor group (see
ESI† for details). As an illustrative example, starting from the
catalytic efficiencies for cyclic carbonate formation.
We recently developed a new class of conformationally
rigid (M-salphen)2 (M = Zn, Pt) luminescent host cavities that
exhibit interesting ion-selective binding behavior.16,17 Instigated
by our ability to change and manipulate the molecular motion
bis-boronic acid of dibenzofuran (Scheme 1), complexes
7 and
8
were prepared by Pd-catalyzed coupling with substituted 5-
and intramolecular
π-π interactions within the cofacial (M-
bromosalicylaldehyde at elevated temperature, followed by a
one-pot templated condensation reaction18 in MeOH using
Zn(OAc)2 and the corresponding “semi-Schiff base” substrate
in the presence of NEt3. Such a stepwise procedure allows the
incorporation of different substituents at the “inner” and “outer”
phenolate groups. In general, these structures afford minimal
lateral displacement but varying extents and propensity for
axial rotation of the Zn-salphen units (e.g. by changing
salphen)2 moiety, we anticipated that such an approach, but
tailored towards reactivity, may be applicable to the
development of (M-salphen)2 complexes as catalytic
frameworks. We thus targeted shape-persistent bimetallic
Department of Biology and Chemistry, City University of Hong Kong,
Tat Chee Avenue, Kowloon, Hong Kong, China. Email:
mcwchan@cityu.edu.hk
† Electronic supplementary information (ESI) available: listings of
experimental details, characterization data and additional NMR spectra;
anchoring position in
1 vs. 2).
crystallographic
data
file
for
7·AcOH
(CIF).
See
DOI: 10.1039/x0xx00000x
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In contrast to the other derivatives, complex
7 displays good
N
N
O
Zn
O
solubility in MeOH and did not precipitate from the Zn-
templated reaction, but instead was recrystallized using
CH2Cl2/hexane to give the acetic acid-bound19 product
7·AcOH, which has been fully characterized including by X-ray
crystallography‡ (ESI). The acetic acid molecule is readily
removed by washing with H2O. Complex 7·AcOH crystallizes
as extended chains of Zn2 frameworks linked by the oxygen
atoms of the acid, with a center of inversion (Fig. 2). A
tetragonal pyramidal geometry is evident for the Zn atom
within each of the non-planar Zn-salphen units, which are
linked to the dibenzofuran bridge in an anti cofacial fashion
R2
R2
N
R1
tBu
O
Zn
N
N
O
O
O
tBu
N
O
R1
N
O
tBu
Zn
N
Zn
tBu
O
R1 = H, R2 = Bu ( )
t
O
1
5
tBu
R1 = tBu, R2 = H ( )
2
( )
tBu
t
6
R
1 = R2 = Bu ( )
R2
N
N
O
R1
Zn
O
tBu
N
O
N
O
Zn
tBu
tBu
R1 = H, R2 = Bu ( )
t
tBu
4
7
R
1 = tBu, R2 = H ( )
O
t
R1 = R2 = Bu ( )
O
8
N
O
N
O
with a dihedral angle (aryl-to-aryl) of 34.3°. The tert-butyl
Zn
tBu
N
groups ortho to the O(phenolate) moieties apparently disfavor a
syn cofacial conformation, in which the pairs of O(phenolate)
atoms would converge as a result of axial rotation.16,17
Intramolecular interplanar separations of ≥ 3.9 Å signify the
absence of π-π interactions. The Zn···Zn distances within the
framework, and bridged by AcOH, are 6.433(1) and 6.065(1) Å
respectively. Although the relative spatial arrangement of the
(Zn-salphen)2 moiety in 7·AcOH may not necessarily
correspond to that of a functioning catalyst, due to the presence
of AcOH bridges plus crystal packing effects, the observed Zn2
cavity can nevertheless be considered as an illustration of
potentially synergistic reactive sites.
N
Zn
O
(3)
R1
O
tBu
R2
N
O
N
O
Zn
tBu
tBu
M1
(
)
Fig. 1 Structures of binuclear complexes 1–8 and mononuclear control M1.
CHO
OH
tBu H2N
CHO
OH
Br
1.
B(OH)2
B(OH)2
N
tBu
HO
R2
Pd(PPh3)4
K2CO3
O
O
tBu
Preliminary evaluation of the binuclear complexes as
catalysts for cyclic carbonate formation, from CO2 and 1,2-
epoxyhexane in 2-butanone (MEK), was undertaken (Table 1).
Zn(OAc)2
NEt3
CHO
OH
DME/H2O,
2. dil. HCl (aq)
MeOH
tBu
Starting from
base ( ) or anchor component (
solubility in MEK) did not increase activity.
1
, changing the anchoring position (
2), Schiff
R2
N
O
3
4, which displays limited
N
O
tBu
Zn
R2 = tBu:
MeOH
(wash)
tBu
R
2 = H
[7 AcOH]
R2 = H (7), tBu (8)
Table 1
Coupling of CO2 and 1,2-epoxyhexane in MEK: evaluation of
O
H2O
(wash)
ligand frameworka,b
N
O
N
Zn
tBu
O
tBu
R2
Scheme 1 Synthesis of dibenzofuran-based complexes 7 and 8.
Conv.c
/%
Conv.c
/%
Run
Cat.
Run
Cat.
1
2
3
4
5
24
20
6
7
8
9
10
27
38
82
87
1
2
3
4
5
6
7·AcOH
8
7
8
M1
19 d
25
55
a
Conditions: 2 mmol epoxyhexane and 0.667 mmol mesitylene (internal
standard) in MEK (5 mL), 10 bar CO2 initial pressure, 45 °C, 2 h. b Catalyst
n
concentration (2.0 mol%; 2.7 mol% Bu4NI co-catalyst) is calculated on per-
metal atom basis and relative to epoxide. c Determined by 1H NMR analysis
of reaction mixture (selectivity >99%). d Not fully dissolved before and after
reaction.
It became clear that elevated catalyst solubility under such
reaction conditions would be beneficial, and tert-butyl
substituents ortho to the “inner” O(phenolate) groups were
therefore introduced, but also to suppress usually facile
dimerization processes (via Zn···O(salphen) interactions16,20) as
well as inhibit the aggregation and thus obstruction of active
sites. Although this resulted in minimal improvements for
5 and
Fig. 2 Perspective view of 7∙AcOH (30% probability ellipsoids).
2 | J. Name., 2012, 00, 1-3
6
, substituting the xanthene bridge for dibenzofuran (thereby
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opening the angle and modifying the distance and orientation
between Zn-salphen units) finally afforded high conversion
The reactions of CO2 in neat epoxide at different catalyst
loadings and temperatures were examined (Table 3). In
levels for
found to give elevated yields (e.g. for
7 and 8
. A slight excess of the nBu4NI co-catalyst was
epoxypropane, catalyst
8 displayed significant activity at low
7, conversion = 64% at
concentrations, and higher turnover frequencies (TOF) per Zn
center were obtained using a larger excess of co-catalyst and at
elevated temperatures (runs 2–4). Hence, a TOF of 2850h–1Zn–1
1:1 (per Zn); 82% at 1:1.35). Lower activity was observed for
7·AcOH, for which increased competition for binding at Zn(II)
would be anticipated. Compared to the mononuclear control
was observed using
8
(0.01 mol%; 0.1 mol% of nBu4NI) at
M1, the enhanced activities for
7 and 8 respectively (on per-Zn
85 °C, which increased to 4300 h–1 Zn–1 for 0.5 mol% of nBu4NI
(runs 4 vs. 7; N.B. run 6 confirms minimal conversion in
basis) are noteworthy.
The performance of the dibenzofuran-based catalyst
8 under
absence of
8). Interestingly, the difference in activity between
(Table 3, runs 4 vs. 5), while less pronounced than in MEK
(Table 1, runs 9 vs. 4; bearing in mind that displays good
8
), and to 5200 h–1 Zn–1 for 0.0025 mol% of
8
(run
various conditions were investigated (Table 2). At 10 bar pCO
,
2
8
and
4
extending the reaction time from 2 to 4 hours gave close to
quantitative yield, while afforded a comparable yield of 92%.
Catalyst was found to give good efficiencies at decreasing
CO2 pressures, with high conversions achieved at 1 bar pCO
7
4
8
solubility in neat epoxide but not MEK), emphasizes the
beneficial impact of the ortho-to-O(phenolate) substituents.
2
and 45
°C for various epoxides in relatively short reaction times
High activity was also observed for
8 in epoxybutane (Table 3,
(runs 3–5), and especially at elevated temperatures (runs 6, 7).
runs 9, 10). Elevated TOF values were achieved in 1,2-
epoxyhexane at 95 °C (runs 11, 12), and lowering the catalyst
loading to 0.0025 mol% gave an initial TOF of 14800 h–1,
Table 2 Coupling of CO2 and epoxide in MEK under various conditionsa,b
1
equivalent to 7400 h–1 per Zn center (see ESI† for H NMR
analysis of reaction mixture for runs 10 and 12).
The initial turnover frequencies observed for
8
approach21
and compare
d,3,8,9,22
the benchmark examples in the literature,2
p
Temp
/°C
45
Time
/h
4
Conv.c
/%
95
CO2
/bar
10
Run
R
Cat.
favorably with respect to previously reported zinc
catalysts6,7,14,15 under similarly mild conditions (at or below 10
1
2
3
4
5
6
7
nBu
nBu
Me
Et
nBu
nBu
nBu
8
8
8
8
8
8
8
5
1
1
1
1
1
45
45
45
45
65
95
4
8
8
8
4
2
89
80
84
87
82
91
bar pCO and below 100 °C). We tentatively postulate that these
2
dibenzofuran-linked shape-persistent structures can confer
suitable distances and geometry between the two Zn centers to
facilitate synergistic (and potentially cooperative) reactivity
with regards to simultaneous activation of epoxide and CO2,
and the subsequent carbonate-forming step. Kinetic and DFT
studies will be performed to probe this possibility.23
In summary, the strategy of modifying the structure and
topology of these frameworks, in order to create Zn2 cavities as
well as adjust their attendant catalytic reactivity, has been
explored. Attractive catalytic performance for coupling of CO2
and epoxide, in terms of high turnover frequencies (per metal
center) and short reaction times under mild conditions (low
a Conditions: 2 mmol epoxide and 0.667 mmol mesitylene (internal standard)
in MEK (5 mL). b As Table 1.
Table 3 Coupling of CO2 and epoxide (neat) under various conditionsa,b
O
Zn cat, nBu4NI
O
O
O
+ CO2
(10 bar)
2 h, neat
R
pCO and temperature), have been achieved. It should be noted
R
2
Cat.
/mol%
8, 0.05
Co-cat. Temp Conv. c TOF d/
/mol%
0.1
that the utilization of abundant, non-toxic Zn is appealing, and
the modification and functionalization of salphen ligands (e.g.
with ammonium/piperidinium moiety) are well-established.
Further investigations regarding substrate scope will be
undertaken. The development of novel, efficient catalytic
reactions mediated by shape-persistent bimetallic assemblies
with tunable topologies may be envisaged.
The work described in this paper was fully supported by a
grant from the Research Grants Council of the Hong Kong
Special Administrative Region, China (CityU 100410).
Run
R
/°C
45
/%
32
h
–1 Zn–1
320
1
2
Me
Me
Me
Me
Me
Me
Me
Me
Et
8, 0.01
8, 0.01
8, 0.01
4, 0.01
–
0.1
45
65
85
85
85
85
85
95
95
95
95
12
44
57
37
2
600
2200
2850
1850
–
3
0.1
4
0.1
5
0.1
6
0.5
7
8, 0.01
8, 0.0025
8, 0.01
8, 0.0025
8, 0.01
8, 0.0025
0.5
86
26
99
30
89
37
4300
5200
4950
6000
4450
7400
8
0.125
0.5
9
10
11
12
Et
0.125
0.5
nBu
nBu
Notes and references
0.125
‡ Crystal data for 7·AcOH: C70H68N4O7Zn2, Mr = 1208.02,
a
Conditions: neat epoxypropane (42.9 mmol), -butane (57.5 mmol) or -
crystal size = 0.24 × 0.22 × 0.15 mm3, orthorhombic, Pnna
,
a
=
hexane (24.9 mmol), 1 mol% mesitylene (internal standard), 10 bar CO2
17.8685(2),
= 7634.92(15) Å3,
mm–1,
= 173(2) K, 2
b
= 23.0557(2),
c
= 18.5326(2) Å,
α
=
β
=
γ
= 90°,
) =1.140
V
initial pressure, 2 h. b Catalyst concentration is calculated on per-metal atom
Z
= 4, Dc = 1.052 g cm–3,
μ
(CuK
α
c
d
basis and relative to epoxide.
mol(substrate converted)/molcat/h/metal).
As Table 1.
Turnover frequency (=
T
θ
range = 6.12 to 143.44º, 7390
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independent reflections (Rint = 0.0284), 396 parameters, R1
0.0537 ( ≥ 2 )), wR2 = 0.1558; CCDC number 1407255.
=
I
σ(I
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21 The Al-[amino-tris(phenolate)] catalyst gave a TOF of 7600
h–1 (ref. 8; 0.0025 mol%, 10 bar
p
, 90
°
C, 2 h); higher
TOF values were reported at significantly lower catalyst
loadings. The [(Al-salen)2( -O)] catalyst was also tested, at
CO2
μ
0.0005 mol% loading under identical conditions, to yield a
TOF of 15000 h–1.
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Tian, Y.-M. Wang, C.-X. Bai, H. Wang and R. Zhang, J. Am.
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23 An additional mechanism invoking simultaneous epoxide and
iodide coordination at the Zn2 centers, leading to accelerated
ring-opening, should also be considered.
4 | J. Name., 2012, 00, 1-3
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