Inorganic Chemistry
Article
by hyper-cross-linking using FCR.10,37 The cross-linked
polymers were incorporated with tetracarbonylcobaltate anions
to produce a Lewis acid−base pair bimetallic heterogeneous
catalyst for the ring-expansion carbonylation of epoxides with
excellent reactivity and selectivity.10 Similarly, an aluminumph-
thalocyanine-based picket fence-type POP was constructed
using solvent knitting FCR to generate a robust network with
good chemical stability and carbonylation activity.38
Herein, a synthetic strategy for constructing metallosalphen-
containing porous organic polymers via direct heterogenization
of metallosalphen monomers using AlCl3- and FeCl3-catalyzed
FCR is demonstrated for the first time. The metallosalphen
POP was further incorporated with [Co(CO)4]− anions to
generate a Lewis acid−base paired structure. Epoxide carbon-
ylation was demonstrated with excellent catalytic activity.
Figure 1. Catalytic applications of metallosalphen complexes in
various reactions.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of POP-
SalphenM(III)Cl. Schiff base salphen catalysts are excellent
candidates for a variety of molecular transformations and
polymerization reactions.9 To date, only a few metallosalphen
POPs have been reported; each requires a tedious synthetic
route.18,39−41 Direct heterogenization of active catalysts by
covalently linking the aromatic rings of homogeneous catalysts
using FCR has gained widespread interest, as it provides a
simple, rapid, inexpensive, and scalable one-pot method for
generating highly porous solid material networks.10,27,37,38,42,43
As a first attempt to heterogenize metallosalphen using FCR
knitting linkages, we synthesized the salphenAl(III)Cl
monomer (1). This monomer was used as a model complex
to study and optimize heterogenization conditions (Scheme
1).
possible causes of the low activity and selectivity and poor
recyclability.19 To overcome these limitations, direct hetero-
genization of the active homogeneous catalytic system is highly
desirable, provided the heterogenization pathway is simple and
inexpensive and is capable of producing robust catalytic
materials that are dense in active sites.10 Various synthetic
strategies can be used for the direct heterogenization of active
homogeneous systems.9 The synthesis of POPs via Friedel−
Crafts reaction (FCR) catalyzed by Lewis acids AlCl3 and
FeCl3 to knit homogeneous catalysts together through covalent
linkages has received considerable attention.20−23 The appeal
of this approach lies in its simplicity.24 With other POP
synthetic routes, specific functional groups must be incorpo-
rated by tedious synthetic routes; in contrast, the FCR method
represents a facile, inexpensive, and scalable approach for
producing highly porous robust networks.25,26 Recently, a
novel phenanthroline-based POP with immobilized iridium
was generated by FCR; the catalyst promotes the hydro-
genation of CO2 to formate under basic conditions with
excellent activity and recyclability.27 In the study presented
here, catalytically important metallosalphen complexes were
used as a constituting monomer to construct a POP using a
cross-linking strategy by simple FCR. The generated metal-
losalphen POPs can be functionalized to offer new
opportunities as a heterogenized catalyst in emerging research
fields such as CO2 polymerization and ring-expansion
carbonylation of epoxides.
Scheme 1. Synthetic Scheme and Reaction Pathways for the
t
Porous SalphenAl(III)chloride Polymer (2) (R = Bu)
Ring-expansion carbonylation is the most widely studied
topic of epoxide transformation because it utilizes carbon
monoxide as a C1 resource to produce chemically important
critical intermediate β-lactones in an atom economic way.28,29
The produced β-lactones have inherent ring strain, which
makes them a highly sought target for producing industrially
important value-added chemicals like poly(β-hydroxyalka-
notes) a biodegradable polymer,30,31 β-hydroxy acids,32
succinic acid, succinic anhydrides,33 acrylic acid, etc.34,35
Despite the growing need for β-lactones, their commercial
production is still a challenging task.36 In this regard, ring-
expansion carbonylation of epoxides is an attractive direct one-
pot method for the production of β-lactones from the
commercially available epoxides. In recent times,
tetraphenylporphyrinat oaluminum(III)Cl and
tetraphenylporphyrinatochromium(III)Cl (TPPAlCl and
TPPCrCl, respectively) monomers were directly heterogenized
To better understand the effect of the cross-linker, reaction
pathway, and porosity on the resulting polymer, we explored
two methods for synthesizing POPs using different cross-
linkers. For our first attempt to synthesize 2, we used
dichloromethane both as a solvent and as an external cross-
linker and AlCl3 as a catalyst for the solvent knitting reaction
method; no solid products were formed, even after refluxing
for 24 h. 1H nuclear magnetic resonance (NMR) spectroscopy
indicated monomer 1 as the main component. However, when
1 was combined with formaldehyde dimethyl acetal (FDA,
CH3OCH2OCH3) as an external cross-linker using AlCl3 as a
catalyst, a dark yellow solid product resulted. The reaction was
1
monitored by H NMR spectroscopy by sampling at regular
intervals. After 1 h, integration of the p protons of the phenolic
moiety of 1 decreased and the spectra indicated that structural
B
Inorg. Chem. XXXX, XXX, XXX−XXX