Self-Assembled Bimetallic Aluminum-Salen Catalyst for the Cyclic Carbonates Synthesis

Bimetallic bis-urea functionalized salen-aluminum catalysts have been developed for cyclic carbonate synthesis from epoxides and CO2. The urea moiety provides a bimetallic scaffold through hydrogen bonding, which expedites the cyclic carbonate formation reaction under mild reaction conditions. The turnover frequency (TOF) of the bis-urea salen Al catalyst is three times higher than that of a μ-oxo-bridged catalyst, and 13 times higher than that of a monomeric salen aluminum catalyst. The bimetallic reaction pathway is suggested based on urea additive studies and kinetic studies. Additionally, the X-ray crystal structure of a bis-urea salen Ni complex supports the self-assembly of the bis-urea salen metal complex through hydrogen bonding.

In some bimetallic catalytic reactions, two metal centers can cooperatively and simultaneously activate both reaction partners (i.e., epoxide and CO 2 ), which leads to second-order reaction kinetics [31]. For example, North group reported bimetallic aluminum catalysts, where the µ-oxo-bridged salen aluminum catalyst showed a 10-fold increase in turnover frequency (TOF) compared to a monometallic salen aluminum catalyst (Figure 1a µ-oxo-bridged catalyst) [19]. In their proposed mechanistic scenarios, Molecules 2021, 26, 4097 2 of 9 one aluminum center was coordinated by ammonium-supported CO 2 and the other was coordinated by the epoxide [19,32]. North and co-workers also demonstrated that the µ-oxo-bridged salen aluminum catalyst remained active in the absence of an ammonium halide cocatalyst [33]. Later, North group developed the single framed bimetallic salen aluminum catalyst (Figure 1a) [34].
1a μ-oxo-bridged catalyst) [19]. In their proposed mechanistic scenarios, one aluminum center was coordinated by ammonium-supported CO2 and the other was coordinated by the epoxide [19,32]. North and co-workers also demonstrated that the μ-oxo-bridged salen aluminum catalyst remained active in the absence of an ammonium halide cocatalyst [33]. Later, North group developed the single framed bimetallic salen aluminum catalyst (Figure 1a) [34].

Catalyst Preparation
Bis-urea salen ligands were synthesized in five steps following a reported procedure. The ligands were prepared from commercially available 2-tert-butylphenol, and the overall yield was 44%. All intermediate compounds were checked by 1 H NMR [35]. Metalation with aluminum chloride was performed by a previously reported procedure that involved We previously reported self-assembling urea-functionalized salen cobalt catalysts for the hydrokinetic resolution of epoxides [35]. Those catalysts can self-assemble in solution through weak hydrogen bonding interactions [31,35]. Herein, we report that the selfassembling bimetallic strategy can also be applied to the salen-aluminum catalysts for the cyclic carbonate synthesis from CO 2 and epoxides [36][37][38][39][40][41].

Catalyst Preparation
Bis-urea salen ligands were synthesized in five steps following a reported procedure. The ligands were prepared from commercially available 2-tert-butylphenol, and the overall yield was 44%. All intermediate compounds were checked by 1 H NMR [35]. Metalation with aluminum chloride was performed by a previously reported procedure that involved

X-ray Crystallography
While we tried to grow a single crystal of a self-assembled bis-urea salen aluminum complexes, crystallization was unsuccessful. Instead, a single crystal of the bis-urea salen nickel complex was obtained by slow evaporation in N,N-dimethylformamide (DMF) (Figure 3, 4). The Ni complex (4) has same ligand as bis-urea salen catalyst (3b). As a packed crystal, the Ni complex shows a parallel head-tail conformation. In this structure, intermolecular hydrogen bonding interactions between urea groups are observed (N-H•••O = 2.06, 2.08 Å) at both ends of the salen, and the two urea planes are significantly twisted (57.9(8)°). The metal-metal distance is measured as 5.3 Å.

X-ray Crystallography
While we tried to grow a single crystal of a self-assembled bis-urea salen aluminum complexes, crystallization was unsuccessful. Instead, a single crystal of the bis-urea salen nickel complex was obtained by slow evaporation in N,N-dimethylformamide (DMF) (Figure 3, 4). The Ni complex (4) has same ligand as bis-urea salen catalyst (3b). As a packed crystal, the Ni complex shows a parallel head-tail conformation. In this structure, intermolecular hydrogen bonding interactions between urea groups are observed (N-H•••O = 2.06, 2.08 Å) at both ends of the salen, and the two urea planes are significantly twisted (57.9(8) • ). The metal-metal distance is measured as 5.3 Å.

X-ray Crystallography
While we tried to grow a single crystal of a self-assembled bis-urea salen aluminum complexes, crystallization was unsuccessful. Instead, a single crystal of the bis-urea salen nickel complex was obtained by slow evaporation in N,N-dimethylformamide (DMF) (Figure 3, 4). The Ni complex (4) has same ligand as bis-urea salen catalyst (3b). As a packed crystal, the Ni complex shows a parallel head-tail conformation. In this structure, intermolecular hydrogen bonding interactions between urea groups are observed (N-H•••O = 2.06, 2.08 Å) at both ends of the salen, and the two urea planes are significantly twisted (57.9(8)°). The metal-metal distance is measured as 5.3 Å.

Optimization of Cyclic Carbonates Synthesis Reaciton Conditions
The initial test reaction used for this study was the conversion of propylene oxi propylene carbonate ( Figure 5). The initial reaction was conducted in a closed system sisting of a Teflon sealed Schlenk flask at 1 bar of CO2 and 45 °C. The bis-urea salen minum catalyst (3b) exhibited a TOF that was 13 times higher than a TOF of a stan salen-aluminum catalyst (3a) with tetrabutylammonium bromide ((n-Bu)4N + Br − ) (Ta Entries 1 and 2). Use of tetrabutylammonium iodide ((n-Bu)4N + I − ) led to slightly h TOF, compared to the use of TAB under the initial reaction conditions (1 bar of CO 45 °C) ( Table 1, Entries 2 and 3). To increase the TOF, the reaction was conducted in mL stainless steel bomb reactor. At 10 bar of CO2 and 90 °C, the bis-urea salen alum catalyst showed a higher TOF than previously reported monometallic salen alum catalyst (Table1, Entries 4 and 5) and bridged salen bimetallic aluminum catalyst (Ta Entries 5 and 6). It is important to note that the urea moiety catalyst improves the regardless of the conditions of the cyclic carbonate synthesis from CO2 and epoxide tion system.

Optimization of Cyclic Carbonates Synthesis Reaciton Conditions
The initial test reaction used for this study was the conversion of propylene oxide to propylene carbonate ( Figure 5). The initial reaction was conducted in a closed system consisting of a Teflon sealed Schlenk flask at 1 bar of CO 2 and 45 • C. The bis-urea salen aluminum catalyst (3b) exhibited a TOF that was 13 times higher than a TOF of a standard salen-aluminum catalyst (3a) with tetrabutylammonium bromide ((n-Bu) 4 N + Br − ) ( Table 1, Entries 1 and 2). Use of tetrabutylammonium iodide ((n-Bu) 4 N + I − ) led to slightly higher TOF, compared to the use of TAB under the initial reaction conditions (1 bar of CO 2 and 45 • C) ( Table 1, Entries 2 and 3). To increase the TOF, the reaction was conducted in a 10 mL stainless steel bomb reactor. At 10 bar of CO 2 and 90 • C, the bis-urea salen aluminum catalyst showed a higher TOF than previously reported monometallic salen aluminum catalyst (Table 1, Entries 4 and 5) and bridged salen bimetallic aluminum catalyst ( Table 1, Entries 5 and 6). It is important to note that the urea moiety catalyst improves the TOF regardless of the conditions of the cyclic carbonate synthesis from CO 2 and epoxide reaction system. metal-metal distance between neighboring dimers is measured as 4.9 Å (Figure crystal structure of the bis-urea salen nickel complex supports the bimetallic sca the bis-urea salen aluminum catalyst.

Optimization of Cyclic Carbonates Synthesis Reaciton Conditions
The initial test reaction used for this study was the conversion of propylene propylene carbonate ( Figure 5). The initial reaction was conducted in a closed syst sisting of a Teflon sealed Schlenk flask at 1 bar of CO2 and 45 °C. The bis-urea sa minum catalyst (3b) exhibited a TOF that was 13 times higher than a TOF of a s salen-aluminum catalyst (3a) with tetrabutylammonium bromide ((n-Bu)4N + Br − ) ( Entries 1 and 2). Use of tetrabutylammonium iodide ((n-Bu)4N + I − ) led to slightly TOF, compared to the use of TAB under the initial reaction conditions (1 bar of C 45 °C) ( Table 1, Entries 2 and 3). To increase the TOF, the reaction was conducted mL stainless steel bomb reactor. At 10 bar of CO2 and 90 °C, the bis-urea salen alu catalyst showed a higher TOF than previously reported monometallic salen alu catalyst (Table1, Entries 4 and 5) and bridged salen bimetallic aluminum catalyst ( Entries 5 and 6). It is important to note that the urea moiety catalyst improves regardless of the conditions of the cyclic carbonate synthesis from CO2 and epoxi tion system.

Origin of Beneficial Urea Effects
In the monomeric salen aluminum catalyst system, 0.08 mol% (4 equivalent with respect to the catalyst) of urea additive was added and tested under the optimized conditions; notably, the TOF did not increase. The free urea in the reaction system did not appear to affect the reaction (Figure 7) ( Table 2, Entries 1 and 2).

Origin of Beneficial Urea Effects
In the monomeric salen aluminum catalyst system, 0.08 mol% (4 equivalent with respect to the catalyst) of urea additive was added and tested under the optimized conditions; notably, the TOF did not increase. The free urea in the reaction system did not appear to affect the reaction (Figure 7) (  Figure 7. Urea additive study. We speculated that the TOF enhancement could occur owing to the bimetallic character enabled by the hydrogen bonding between urea groups of the ligand. Thus, we plotted a reaction rate vs. the changes in the amount of catalyst 3b. The graph showed a clear second-order functional graph (R 2 = 0.9977), suggesting a bimetallic reaction pathway ( Figure 8) [35]. FTIR spectroscopy study was conducted to obtain more direct experimental evidence for self-association through urea-urea hydrogen bonding in solution. Bis-urea salen Al catalyst (3b) in THF was measured by FTIR at 25 °C. The FTIR experiments revealed the intensity of hydrogen bonded NH stretching vibration ( = 3444 cm −1 ) increased with increasing concentration, and the intensity of free NH stretching vibration ( = 3966 cm −1 and 3808 cm −1 ) decreased with increasing concentration. The results of FTIR experiments suggest intermolecular hydrogen bonding between bis-urea salen Al complexes in THF solution. (Figure 9. See Supplementary Materials for more details.) [35].   We speculated that the TOF enhancement could occur owing to the bimetallic character enabled by the hydrogen bonding between urea groups of the ligand. Thus, we plotted a reaction rate vs. the changes in the amount of catalyst 3b. The graph showed a clear second-order functional graph (R 2 = 0.9977), suggesting a bimetallic reaction pathway ( Figure 8) [35].   We speculated that the TOF enhancement could occur owing to the acter enabled by the hydrogen bonding between urea groups of the ligand ted a reaction rate vs. the changes in the amount of catalyst 3b. The graph second-order functional graph (R 2 = 0.9977), suggesting a bimetallic re ( Figure 8) [35]. FTIR spectroscopy study was conducted to obtain more direct experi for self-association through urea-urea hydrogen bonding in solution. B catalyst (3b) in THF was measured by FTIR at 25 °C. The FTIR experime intensity of hydrogen bonded NH stretching vibration ( = 3444 cm −1 ) in creasing concentration, and the intensity of free NH stretching vibration and 3808 cm −1 ) decreased with increasing concentration. The results of FT suggest intermolecular hydrogen bonding between bis-urea salen Al co solution. (Figure 9. See Supplementary Materials for more details.) [35]. FTIR spectroscopy study was conducted to obtain more direct experimental evidence for self-association through urea-urea hydrogen bonding in solution. Bis-urea salen Al catalyst (3b) in THF was measured by FTIR at 25 • C. The FTIR experiments revealed the intensity of hydrogen bonded NH stretching vibration ( ν = 3444 cm −1 ) increased with increasing concentration, and the intensity of free NH stretching vibration ( ν = 3966 cm −1 and 3808 cm −1 ) decreased with increasing concentration. The results of FTIR experiments suggest intermolecular hydrogen bonding between bis-urea salen Al complexes in THF solution (Figure 9 See Supplementary Materials for more details) [35].  Figure 9. The NH stretching region of the FTIR spectra of 3b in THF at two different (1 mM (grey), 1.5 mM (black)) at 25 °C.

Conclusions
In conclusion, the self-assembly strategy was successfully applied to the ized salen-aluminum catalysts for cyclic carbonate synthesis reaction from CO ide. The bis-urea functionalized salen ligands were designed to self-assem urea-urea hydrogen bonding. The bimetallic urea salen aluminum complex improved reaction rate (up to 13 times at 1 bar of CO2, 45 °C and up to 2.2 tim of CO2, 90 °C) in the cyclic carbonate formation from epoxides with CO2. Fre tive studies and kinetic studies were performed to confirm bimetallic reactio FTIR spectroscopy study provided an experimental evidence for urea-ure bonding in solution. This work demonstrates that hydrogen bonding can b the catalysts for cyclic carbonate synthesis reaction. It is important to note t moiety improves the TOF regardless of the reaction conditions. Modification and structures to further improve the catalyst are currently in progress.

Conclusions
In conclusion, the self-assembly strategy was successfully applied to the functionalized salen-aluminum catalysts for cyclic carbonate synthesis reaction from CO 2 and epoxide. The bis-urea functionalized salen ligands were designed to self-assemble through urea-urea hydrogen bonding. The bimetallic urea salen aluminum complex showed an improved reaction rate (up to 13 times at 1 bar of CO 2 , 45 • C and up to 2.2 times at 10 bar of CO 2 , 90 • C) in the cyclic carbonate formation from epoxides with CO 2 . Free urea additive studies and kinetic studies were performed to confirm bimetallic reaction pathway. FTIR spectroscopy study provided an experimental evidence for urea-urea hydrogen bonding in solution. This work demonstrates that hydrogen bonding can be applied to the catalysts for cyclic carbonate synthesis reaction. It is important to note that the urea moiety improves the TOF regardless of the reaction conditions. Modifications of the ligand structures to further improve the catalyst are currently in progress.