Construction of a (NNN)Ru-Incorporated Porous Organic Polymer with High Catalytic Activity for β-Alkylation of Secondary Alcohols with Primary Alcohols

Solid supports functionalized with molecular metal catalysts combine many of the advantages of heterogeneous and homogeneous catalysis. A (NNN)Ru-incorporated porous organic polymer (POP-bp/bbpRuCl3) exhibited high catalytic efficiency and broad functional group tolerance in the C–C cross-coupling of secondary and primary alcohols to give β-alkylated secondary alcohols. This catalyst demonstrated excellent durability during successive recycling without leaching of Ru which is ascribed to the strong binding of the pincer ligands to the metal ions.


Synthesis of POP-bp/bbpRuCl 3
Under a N 2 atmosphere, a mixture of POP-bp/bbp (0.2 g), RuCl 3 ·xH 2 O (0.017 g), and anhydrous ethanol (25 mL) was introduced into a round-bottom flask (100 mL). The resulting mixture was refluxed for 12 h. After this period, the brown solid POP-bp/bbpRuCl 3 was separated by centrifugation, washed sequentially with ethanol and ethyl ether, and then dried under vacuum.

Typical Procedure for Syntheses of β-Alkylated Secondary Alcohols
Under an N 2 atmosphere, a mixture of secondary alcohol (1 mmol), primary alcohol (1.2 mmol), POP-bp/bbpRuCl 3 (20 mg, 0.6 mol% Ru), KOH (0.5 mmol) and toluene (2 mL) was added into a 15 mL sealed tube equipped with a stirring bar. The reaction mixture was heated to 130 • C for 12 h. After cooling to ambient temperature, the catalyst was separated by centrifugation and washed with ethanol and diethyl ether. The catalyst was then dried in a vacuum at 60 • C for 2 h to give recycled catalyst for the next run. The organic layers were combined and dried over anhydrous Na 2 SO 4 and concentrated under Polymers 2022, 14, 231 3 of 13 reduced pressure. The crude product was purified by flash column chromatography using petroleum ether and ethyl acetate as the eluent.

Characterizations
The analytical instruments employed in this work are as described in our previous article [57,64,65], unless otherwise noted. 1 H and 13 C NMR spectra were recorded at ambient temperature on a Varian UNITY plus-400 spectrometer. Solid-state cross-polarization magic angle spinning 13 C NMR measurements were carried out on Bruker Avance III/WB solid-state NMR spectrometer operating at 400 MHz equipped with a standard 4 mm magic angle spinning double resonance probe head. Powder X-ray diffraction patterns were collected on a PANalytical Aeris diffractometer (Cu-Ka). Infrared spectra were recorded on a Varian Scamiter-1000 spectrometer. The thermal stability of materials was evaluated using thermogravimetric analysis (Perkin-Elmer Pyrisl) under a nitrogen atmosphere. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy spectra were obtained using a HITACHI S-4700 cold field-emission SEM. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G20 electron microscope operating at 200 kV. Annular dark-field scanning TEM (ADF-STEM) was performed on a FEI Tecnai F20 electron microscope operating at 200 kV, equipped with Genesis EDS detector. X-ray photoelectron spectra were recorded on an X-ray photoelectron spectrometer (AXIS Ultra DLD) and binding energies were referenced to C 1s at 284.7 eV from hydrocarbon to compensate for possible charging effects.

Subsection Synthesis and Characterization of POP-bp/bbpRuCl 3
POP-bp/bbp was synthesized by the AlCl 3 -promoted Scholl reaction of bbp and biphenyl (bp) in CHCl 3 at 58 • C (Scheme 1). The resulting brown polymer was insoluble in common organic solvents such as tetrahydrofuran (THF), methanol (MeOH), N,Ndimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Thermogravimetric analysis (TGA) showed that POP-bp/bbp was stable at temperatures up to 300 • C ( Figure S1). The 13 C NMR solid-state spectrum of POP-bp/bbp contained signals at δ = 148, 140 and 138 ppm ascribed to the carbons on pyridine and imidazole rings, and a broad signal at δ = 125 ppm assigned to the other aromatic carbons ( Figure S2). Scanning electron microscopy (SEM) ( Figure S3) and transmission electron microscopy (TEM) ( Figure S4) images showed that the as-prepared polymer POP-bp/bbp was amorphous with particles of irregular shape and size. The energy-dispersive X-ray (EDX) elemental mapping images ( Figure S5) indicated the homogenous distribution of C and N, indicating homogeneous distribution of bbp monomer. The FT-IR spectrum ( Figure 1a) contained absorptions at 1600, 1573, and 1460 cm −1 attributed to the C−C, C=N and C−N stretching vibrations, respectively. In addition, the band at 3185 cm −1 were assigned to the −NH, originating from the imidazole moiety.  POP-bp/bbp bearing NNN pincer groups was treated with RuCl3·xH2O in refluxing EtOH under N2 for 12 h to give Ru-metalated POP-bp/bbp (POP-bp/bbpRuCl3). The porosity of POP-bp/bbp and POP-bp/bbpRuCl3 was assessed by N2 adsorption and desorption analyses at 77.3 K. Absorption isotherms ( Figure 1b) exhibited a type I adsorption isotherm with steep N2 uptake at low relative pressure (P/P0 < 0.1), indicating abundant micropores in the polymer structure. A slight hysteresis loop with a small rise in N2 uptake at higher pressures was attributed to the presence of mesopores. The Brunauer−Emmett−Teller (BET) surface areas of POP-bp/bbp and POP-bp/bbpRuCl3 were calculated to be 632 and 602 m 2 g −1 , respectively. The pore-size distribution of POP-bp/bbp and POPbp/bbpRuCl3 were estimated with the aid of non-local density functional theory modeling to be centered around 6−10 Å ( Figure S7). Inductively coupled plasma optical emission spectrometry (ICP-OES) of POP-bp/bbpRuCl3 indicated 3.11 wt % Ru loading. The powder X-ray diffraction (PXRD) patterns confirmed that both the polymer and POPbp/bbpRuCl3 were amorphous ( Figure S8).
Ruthenium ion interaction with the NNN pincer moieties were observed by XPS (Figures S9 and S10). Coordination of Ru(III) ions with POP-bp/bbp significantly changes the electron density at the pyridinic N sites and thus the binding energies of their N 1s electrons. The N 1s XPS profile of POP-bp/bbp was deconvoluted into two peaks centered at 398.84 and 400.47 eV, which are assigned to pyridinic N and imidazolic N, respectively ( Figure 1c). After the loading of RuCl3, a new N 1s XPS peak was observed at 399.71 eV, POP-bp/bbp bearing NNN pincer groups was treated with RuCl 3 ·xH 2 O in refluxing EtOH under N 2 for 12 h to give Ru-metalated POP-bp/bbp (POP-bp/bbpRuCl 3 ). The porosity of POP-bp/bbp and POP-bp/bbpRuCl 3 was assessed by N 2 adsorption and desorption analyses at 77.3 K. Absorption isotherms ( Figure 1b) exhibited a type I adsorption isotherm with steep N 2 uptake at low relative pressure (P/P 0 < 0.1), indicating abundant micropores in the polymer structure. A slight hysteresis loop with a small rise in N 2 uptake at higher pressures was attributed to the presence of mesopores. The Brunauer−Emmett−Teller (BET) surface areas of POP-bp/bbp and POP-bp/bbpRuCl 3 were calculated to be 632 and 602 m 2 g −1 , respectively. The pore-size distribution of POP-bp/bbp and POP-bp/bbpRuCl 3 were estimated with the aid of non-local density functional theory modeling to be centered around 6−10 Å ( Figure S7). Inductively coupled plasma optical emission spectrometry (ICP-OES) of POP-bp/bbpRuCl 3 indicated 3.11 wt % Ru loading. The powder X-ray diffraction (PXRD) patterns confirmed that both the polymer and POP-bp/bbpRuCl 3 were amorphous ( Figure S8).
Ruthenium ion interaction with the NNN pincer moieties were observed by XPS ( Figures S9 and S10). Coordination of Ru(III) ions with POP-bp/bbp significantly changes the electron density at the pyridinic N sites and thus the binding energies of their N 1s electrons. The N 1s XPS profile of POP-bp/bbp was deconvoluted into two peaks centered at 398.84 and 400.47 eV, which are assigned to pyridinic N and imidazolic N, respectively ( Figure 1c). After the loading of RuCl 3 , a new N 1s XPS peak was observed at 399.71 eV, corresponding to Ru-bound N species [66]. The binding energy of Ru 3p 3/2 was 463.35 eV, indicating that the Ru species maintained its original oxidation state of Ru 3+ in POP-bp/bbpRuCl 3 [67]. Compared with the reported binding energy of Ru 3p 3/2 of RuCl 3 (464.1 eV), the down-shift (0.75 eV) of coordinated Ru 3+ can be attributed to additional electron density from the strongly electron-donating ligands [68]. High-resolution transmission electron microscopy (HR-TEM) (Figure 1e) did not reveal the presence of Ru nanoparticles. EDX elemental mapping confirmed that the Ru was distributed evenly throughout the POP-bp/bbpRuCl 3 (Figure 1f). The C, N and Ru contents of the as-synthesized POPbp/bbpRuCl 3 were 80.5 wt %, 14.6 wt %, and 4.8 wt %, respectively ( Figure S11). The Ru content obtained by EDX element mapping analysis is higher than that calculated by the ICP-OES analysis, which is attributed to the fact that Ru is mainly loaded on the surface of bp/bbp material.

Catalytic β-Alkylation of Secondary Alcohols
We next determined the performance of as-prepared POP-bp/bbpRuCl 3 as a catalyst for the catalytic acceptorless dehydrogenation coupling of alcohols. The reaction of 1phenylethanol (mol %) with benzyl alcohol (2a) was conducted in toluene with KOH under a nitrogen atmosphere (Table 1). A standard workup produced 1-([1,1 -biphenyl]-4-yl)ethan-1-one (3aa) in 68% yield with a trace of 1,3-diphenylpropan-1-one (4aa) as determined by high-performance liquid chromatography (HPLC). The screening of different bases (entries 1-5) revealed that CsOH was more selective than KOBu t , Cs 2 CO 3 or NaOH, but that KOH facilitated the highest yield. Lowering the reaction temperature to 100 • C or 120 • C reduced the yield of 3aa significantly, while raising it slightly (140 • C) had a modest impact (entries 6-9). The optimal amount of KOH was 0.5 equivalents (entries 10-13). The yield of 3aa could be increased up to 93% by extending the reaction time to 12 h (entries 15). Interestingly, neither of the individual components of the catalyst (i.e., POP-bp/bbp or RuCl 3 ) facilitated more than a very modest amount of product under these conditions (entries 16 and 17). bp/bbpRuCl3 were 80.5 wt %, 14.6 wt %, and 4.8 wt %, respectively ( Figure S11). content obtained by EDX element mapping analysis is higher than that calculated ICP-OES analysis, which is attributed to the fact that Ru is mainly loaded on the of bp/bbp material.

Entry a Secondary Alcohol Product Yield (%)
The recycling of POP-bp/bbpRuCl3 was examined under optimized reaction conditions. No significant loss of catalytic activity was observed after four cycles (Figure 2a) and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not observed in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogeneous distribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12). The FT-IR The recycling of POP-bp/bbpRuCl3 was examined under optimized reaction conditions. No significant loss of catalytic activity was observed after four cycles (Figure 2a) and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not observed in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogeneous distribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12) The recycling of POP-bp/bbpRuCl3 was examined under optimized reaction conditions. No significant loss of catalytic activity was observed after four cycles (Figure 2a) and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not observed in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogeneous distribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12) The recycling of POP-bp/bbpRuCl3 was examined under optimized reaction conditions. No significant loss of catalytic activity was observed after four cycles (Figure 2a) and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not observed in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogeneous distribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12) The recycling of POP-bp/bbpRuCl3 was examined under optimized reaction conditions. No significant loss of catalytic activity was observed after four cycles (Figure 2a) and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not observed in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogeneous distribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12) The recycling of POP-bp/bbpRuCl3 was examined under optimized reaction conditions. No significant loss of catalytic activity was observed after four cycles (Figure 2a) and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not observed in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogeneous distribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12). The FT-IR The recycling of POP-bp/bbpRuCl 3 was examined under optimized reaction conditions. No significant loss of catalytic activity was observed after four cycles (Figure 2a) and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not observed in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogeneous distribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12). The FT-IR spectrum of the recovered catalyst did not change noticeably (Figure 2c), except for a new peak at 1960 cm −1 which we assign to a Ru-H species [69] generated during the catalytic cycle. XPS measurements indicated that the Cl 2p peak had disappeared in the recovered catalyst indicating ligand substitution ( Figure S13). The binding energies of Ru 3p 3/2 and Ru 3d 5/2 for the catalyst were 463.15 eV and 281.4 eV, respectively (Figure 2d and Figure S14). The slight decrease in binding energies compared to that of fresh catalyst may be due to the substitution of hydride, which has a greater electron donating ability than chloride [69]. To verify whether the observed catalysis was due to the heterogeneous catalyst POP-bp/bbpRuCl 3 or due to leached ruthenium species, a reaction was performed between 1a and 2a under standard conditions. The yield of 3aa was 42% accompanied by 7% 4aa after 2h. The reaction was then filtered. No catalytic function was observed in the filtered solution over 24 h and negligible Ru content was detected by ICP. These results indicate that the POP-bp/bbpRuCl 3 catalyst is stable, which we ascribe to the strong binding of pincer ligand to metal centers. a Reaction condition: secondary alcohol (1 mmol), 2a (1.2 mmol), POP-bp/bbpRuCl3 (20 m mol%Ru), KOH (0.5 equiv.), toluene (2 mL), 130 °C, 12 h, isolated yields.
The recycling of POP-bp/bbpRuCl3 was examined under optimized reactio tions. No significant loss of catalytic activity was observed after four cycles (Fi and ICP analysis indicated that 96% of Ru remained. Ru nanoparticles were not o in the subsequent TEM images (Figure 2b). EDX analysis revealed the homogene tribution of Ru components throughout the polymer POP-bp/bbp ( Figure S12). T spectrum of the recovered catalyst did not change noticeably (Figure 2c), except f peak at 1960 cm −1 which we assign to a Ru-H species [69] generated during the cycle. XPS measurements indicated that the Cl 2p peak had disappeared in the re catalyst indicating ligand substitution ( Figure S13). The binding energies of Ru 3 Ru 3d5/2 for the catalyst were 463.15 eV and 281.4 eV, respectively (Figures 2d a The slight decrease in binding energies compared to that of fresh catalyst may b the substitution of hydride, which has a greater electron donating ability than [69]. To verify whether the observed catalysis was due to the heterogeneous cataly bp/bbpRuCl3 or due to leached ruthenium species, a reaction was performed bet and 2a under standard conditions. The yield of 3aa was 42% accompanied by 7% 2h. The reaction was then filtered. No catalytic function was observed in the filter tion over 24 h and negligible Ru content was detected by ICP. These results indi the POP-bp/bbpRuCl3 catalyst is stable, which we ascribe to the strong binding o ligand to metal centers. A probable mechanism of this reaction was determined by investigating each step individually. The POP-bp/bbpRuCl 3 -catalysed dehydrogenation of phenylmethanol and 1-phenylethanol for 4 h afforded benzaldehyde in 90% yield and acetophenone in 73% yield, respectively (Scheme 2(a,b)). The condensation of benzaldehyde with acetophenone, facilitated by KOH afforded chalcone in good yield (Scheme 2(c)). reduction of chalcone with 1a under the standard reaction conditions achieved 31% of the corresponding secondary alcohol. as shown in Scheme 2, hydrogenation of chalcone with 2a under the same reaction conditions also gave the same reduced product in a similar 27% yield (Scheme 2(d)).

Scheme 2. Mechanism elucidation experiments.
In light of these results, and of previous literature reports [14][15][16][17], a catalytic mechanism is proposed (Scheme 3). Primary and secondary alcohols are dehydrogenated by the Ru catalyst to form the corresponding aldehyde, ketone and a ruthenium hydride complex. Base-catalysed aldol condensation of the resulting ketone and aldehyde give the α,β-unsaturated ketone intermediate which is reduced by the ruthenium hydride to generate β-alkylated secondary alcohol.
individually. The POP-bp/bbpRuCl3-catalysed dehydrogenation of phenyl 1-phenylethanol for 4 h afforded benzaldehyde in 90% yield and acetoph yield, respectively (Equations (1) and (2), Scheme 2). The condensation of with acetophenone, facilitated by KOH afforded chalcone in good yield ( reduction of chalcone with 1a under the standard reaction conditions achiev corresponding secondary alcohol. as shown in scheme 2, hydrogenation of 2a under the same reaction conditions also gave the same reduced produ 27% yield (Equation (4)).

Scheme 2. Mechanism elucidation experiments.
In light of these results, and of previous literature reports [14][15][16][17], a ca nism is proposed (Scheme 3). Primary and secondary alcohols are dehydrog Ru catalyst to form the corresponding aldehyde, ketone and a ruthenium plex. Base-catalysed aldol condensation of the resulting ketone and aldehyd unsaturated ketone intermediate which is reduced by the ruthenium hydri β-alkylated secondary alcohol.

Conclusions
A novel porous organic polymer incorporating a NNN pincer ligand cellent thermal durability and high surface area. This acceptorless deh Scheme 3. Proposed mechanism.

Conclusions
A novel porous organic polymer incorporating a NNN pincer ligand possessed excellent thermal durability and high surface area. This acceptorless dehydrogenation, crosscoupling catalyst exhibited good activity, broad substrate scope and good recycling ability. We believe this work provides a green, convenient and scalable method for constructing C-C bonds, combining many of the advantages of homogeneous and heterogeneous catalysis.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.