Hydrogenative coupling of nitriles with diamines to benzimidazoles using lignin-derived Rh2P catalyst

•Nitrile was developed as synthetic building block for organic synthesis•Reductive coupling of nitriles to 1,2-phenylenediamines yielded benzimidazoles•Strong P−Rh interaction and charge transfer to Rh enhanced Rh2P activity•H/D exchange between H2 and –CD3 in CD3CN occurred via intramolecular D-shift Nitrile (C≡N bond) activation for direct organic synthesis has been less explored so far due to a high redox potential of nitrile and its low dissociation energy of C−CN bond. Herein, we demonstrate a direct reductive coupling of nitriles and 1,2-phenylenediamines to yield various benzimidazoles in excellent yields (95%–99%) by using rhodium phosphide (Rh2P) catalyst supported on lignin-derived carbon (LC) using H2 (or hydrazine hydrate) as a hydrogen source. The high catalytic performance of Rh2P/LC is attributed to enhanced charge transfer to Rh and strong P−Rh interactions. Our isotope trace experiment confirms the presence of H/D exchange between H2 and the inert –CD3 group of CD3CN via an intramolecular D-shift. Reusability of Rh2P/LC is further demonstrated by a seven-time recycling without evident loss of activity. This research thus highlights a great potential in organic transformation with nitrile as a synthetic building block. Nitrile (C≡N bond) activation for direct organic synthesis has been less explored so far due to a high redox potential of nitrile and its low dissociation energy of C−CN bond. Herein, we demonstrate a direct reductive coupling of nitriles and 1,2-phenylenediamines to yield various benzimidazoles in excellent yields (95%–99%) by using rhodium phosphide (Rh2P) catalyst supported on lignin-derived carbon (LC) using H2 (or hydrazine hydrate) as a hydrogen source. The high catalytic performance of Rh2P/LC is attributed to enhanced charge transfer to Rh and strong P−Rh interactions. Our isotope trace experiment confirms the presence of H/D exchange between H2 and the inert –CD3 group of CD3CN via an intramolecular D-shift. Reusability of Rh2P/LC is further demonstrated by a seven-time recycling without evident loss of activity. This research thus highlights a great potential in organic transformation with nitrile as a synthetic building block. Benzimidazole and its derivatives are pharmaceutically important heterocyclic compounds with a broad range of biological activities and pharmacological properties such as anti-viral, anti-fungal, anti-bacterial, anti-ulcer, anti-inflammatory, anti-hypertensive, anti-histaminic, anti-cancer, anti-tumor, and anti-HIV features (Chakrabarti et al., 2019Chakrabarti K. Maji M. Kundu S. Cooperative iridium complex catalyzed synthesis of quinoxalines, benzimidazoles and quinazolines in water.Green. Chem. 2019; 21: 1999-2004Crossref Google Scholar; Keri et al., 2015Keri R.S. Hiremathad A. Budagumpi S. Nagaraja B.M. Comprehensive review in current developments of benzimidazole-based medicinal chemistry.Chem. Biol. Drug Des. 2015; 86: 19-65Crossref PubMed Scopus (127) Google Scholar). Their synthetic methods have received extensive attention due to their pharmaceutical importance. Benzimidazoles are traditionally prepared according to Ladenburg ring closure method (Scheme 1A) by direct condensation of 1,2-phenylenediamines (1) with carboxylic acids and their derivatives such as acids, acyl chlorides, anhydrides, aldehydes, amides and nitriles in the presence of strong acid at high reaction temperature. Among these carboxylic acid derivatives, due to a facile access to nitriles and their high availability as commodity chemicals, direct condensation of 1 with nitriles should have great potential in synthetic chemistry of benzimidazoles and in the productions of benzimidazole-related agrochemicals and pharmaceuticals (Dalziel et al., 2018Dalziel M.E. Deichert J.A. Carrera D.E. Beaudry D. Han C. 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Chem. 1944; 9: 31-49Crossref Scopus (46) Google Scholar), 27% yield of 2-methylbenzimidazole (3a) was obtained from 1,2-phenylenediamine (1a) and acetonitrile (2a) in the presence of an equivalent of anhydrous hydrogen chloride (HCl) in a sealed tube at 200°C for six hours (Scheme 1C). The corresponding condensation reaction mechanism suggests an initial formation of highly reactive ammonoacyl chloride (4, Scheme 2A) from nitrile (2) and HCl under anhydrous conditions. The in situ formed 4 smoothly promotes subsequent ring closure to yield benzimidazoles (3). Therefore, nitrile activation, via additively uniting with HCl (4, Scheme 2A) at 200°C, is a prerequisite for the condensation reaction. However, nitrile (C≡N bond) activation has been less explored so far when compared with C=C, C=O, C=N and O−N=O bonds owing to the high redox potential of nitriles and the low dissociation energy of C−CN bond. Most recently, nitrile hydrogenation was developed for atom-economic synthesis of amine with transition-metal-based catalysts (Bagal and Bhanage, 2015Bagal D.B. Bhanage B.M. Recent advances in transition metal-catalyzed hydrogenation of nitriles.Adv. Synth. Catal. 2015; 357: 883-900Crossref Scopus (149) Google Scholar; Werkmeister et al., 2014Werkmeister S. Junge K. Beller M. Catalytic hydrogenation of carboxylic acid esters, amides, and nitriles with homogeneous catalysts.Org. Process. Res. Dev. 2014; 18: 289-302Crossref Scopus (275) Google Scholar; Liu et al., 2018aLiu L. Liu Y. Ai Y. Li J. Zhou J. Fan Z. Bao H. Jiang R. Hu Z. Wang J. et al.Pd-CuFe catalyst for transfer hydrogenation of nitriles: controllable selectivity to primary amines and secondary amines.iScience. 2018; 8: 61-73Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar; Nandi et al., 2017Nandi S. Patel P. Jakhar A. Khan N.H. Biradar A.V. Kureshy R.I. Bajaj H.C. Cucurbit[6]uril-stabilized palladium nanoparticles as a highly active catalyst for chemoselective hydrogenation of various reducible groups in aqueous media.ChemistrySelect. 2017; 2: 9911-9919Crossref Scopus (23) Google Scholar) (Table S1). However, a crucial selectivity issue arises from an inevitable formation of mixtures of primary (7), secondary (9) and tertiary amines (10) with alkylimines (6) and dialkylimines (8) as the proposed reactive intermediates (Scheme 2B). High reactivity of these reaction intermediates (6 and 8) competitively induces a series of parallel and consecutive reactions, resulting in a challenge of selectivity control and product separation. Generally, nitrile hydrogenation can be performed under relatively mild reaction conditions (20–140°C). We thus think that the presence of 1 in the nitrile-hydrogenation system should be able to trap the in situ formed two reactive imines intermediates (6 and 8) to give N-(o-aminopheny)-imine intermediate (11, Scheme 2B). A subsequent cyclization of 11 and successive dehydrogenation of the resulting ring-closing product (12, Scheme 2B) should yield 3 under the reaction conditions (Scheme 2B). Therefore, a transition-metal-promoted reductive coupling of nitriles and 1,2-phenylenediamines was investigated in this research for green and atom-economic synthesis of benzimidazoles (Scheme 2B). As shown in Scheme 2B, the reductive coupling process should be initialized from catalytic hydrogenation of nitriles. Both heterogeneous and homogeneous catalysts were reported for nitriles hydrogenation. Precious metal-complex-based homogeneous catalysts evidently show excellent catalytic performance on the hydrogenation (Bagal and Bhanage, 2015Bagal D.B. Bhanage B.M. Recent advances in transition metal-catalyzed hydrogenation of nitriles.Adv. Synth. Catal. 2015; 357: 883-900Crossref Scopus (149) Google Scholar; Werkmeister et al., 2014Werkmeister S. Junge K. Beller M. 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The lignin was selected as precursor of the catalyst support due to its unique structure of three-dimensional and porous framework, rich in carbon-oxygen-based functional groups on the framework, scalable and renewable nature as a carbon-based feedstock (Zhu et al., 2019Zhu Y. Li Z. Chen J. Applications of lignin-derived catalysts for green synthesis.Green. Energy Environ. 2019; 4: 210-244Crossref Scopus (43) Google Scholar; Chatterjee and Saito, 2015Chatterjee S. Saito T. Lignin-derived advanced carbon materials.ChemSusChem. 2015; 8: 3941-3958Crossref PubMed Scopus (160) Google Scholar). The developed Rh2P/LC demonstrates high efficiency by coupling serial tandem reactions of nitrile hydrogenation, 11 cyclization and 12 dehydration in one-pot (Scheme 2B). In contrast to traditional condensation method (Feng et al., 2016Feng F. Ye J. Cheng Z. Xu X. Zhang Q. Ma L. Lu C. Li X. 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A simple one pot nano titania mediated green synthesis of 2-alkylbenzimidazoles and indazole from aromatic azides under UV and solar light.Catal. Commun. 2009; 11: 280-284Crossref Scopus (31) Google Scholar) (Table S2), Rh2P/LC promoted reductive coupling of 1a and 2a can readily perform at 140°C with >99% yield of 3a by using H2 or hydrazine hydrate (N2H4⋅H2O) as hydrogen sources (Scheme 1C). In this research, LC was used as catalyst support, which was obtained by calcination a mixture of enzymatic hydrolysis lignin (EHL) and potassium bicarbonate (KHCO3) at 800°C under atmospheric N2, followed by thoroughly leaching with aqueous HCl solution. While, Rh2P/LC400 catalyst was prepared by co-loading RhCl3 and bis(diphenylphosphino)ethane (dppe) ligand on the resulting LC surface, followed by a pyrolysis at 400°C under atmospheric H2/N2. The subscript 400 in Rh2P/LC400 indicates the final calcination temperature for catalyst preparation. The introduced dppe ligand provides P source for the formation of Rh2P species on the LC support during calcination procedure. For comparison, Rh catalyst (Rh/LC400) supported on LC was prepared without addition of the dppe ligand to investigate the electronic effect of the introduced P on the catalytic performance of the resulting Rh2P catalyst. Moreover, to understand the influence of metallic site on the reductive coupling, Pd (Pd/LC400) and Ru (Ru/LC400) catalysts were synthesized with same method to Rh2P/LC400; however, the expected metal phosphides of Pd-P and Ru-P samples were undetected on the resulting catalyst surfaces. The morphology of the obtained samples was initially characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). EHL shows a bulky, solid, and compact architecture with irregular shapes and rough surface based on the SEM analysis (Figure S1A). While, the resulting LC exhibits a sponge-like structure with readily accessible, highly crosslinked and randomly opened macropores (Figures S1B and S1C). After Rh2P loading, the obtained Rh2P/LC400 possesses similar SEM micrograph to that of LC (Figure S1D). The TEM of Rh2P/LC400 exhibits ultrathin carbon nanosheet-assembled three-dimensional (3D) network with a crumpled, wrinkled, and rippled structure (Figure 2A). Additionally, Rh, P, and C elements are homogeneously and highly dispersed on the detected area of LC surface as shown by the TEM energy-dispersive X-ray (EDX) images of Rh2P/LC400 (Figure 2M–2P). The estimated average nanoparticle size of Rh2P was 4.3 nm (Figure 2B) with a detectable crystal fringe spacing of 0.280 nm, corresponding to the (200) crystal plane of Rh2P (Su et al., 2020Su J. Zhao H. Fu W. Tian W. Yang X. Zhang H. Ling F. Wang Y. Fine rhodium phosphides nanoparticles embedded in N, P dual-doped carbon film: new efficient electrocatalysts for ambient nitrogen fixation.Appl. Catal. B Environ. 2020; 265: 118589-118596Crossref Scopus (39) Google Scholar) (Figure 2C). In the case of Rh/LC400, its TEM images reveal a mean Rh nanoparticle size of 7.4 nm (Figures 2D and 2E) with lattice fringe spacing of 0.220 nm (Figure 2F), which can be indexed to the (111) plane of the Rh (Su et al., 2020Su J. Zhao H. Fu W. Tian W. Yang X. Zhang H. Ling F. Wang Y. Fine rhodium phosphides nanoparticles embedded in N, P dual-doped carbon film: new efficient electrocatalysts for ambient nitrogen fixation.Appl. Catal. B Environ. 2020; 265: 118589-118596Crossref Scopus (39) Google Scholar). The TEM images of Pd/LC400 indicate a significantly increased average size to 18.3 nm for Pd nanoparticle on the LC support (Figures 2G and 2H). The observed lattice fringe spacing was detected as 0.224 nm, belonging to the (111) plane of Pd (Sun et al., 2020Sun W. Wu S. Lu Y. Wang Y. Cao Q. Fang W. Effective control of particle size and electron density of Pd/C and Sn-Pd/C nanocatalysts for vanillin production via base-free oxidation.ACS Catal. 2020; 10: 7699-7709Crossref Scopus (20) Google Scholar) (Figure 2I). Finally, for Ru/LC400 sample, TEM images show an average nanoparticle size of 3.3 nm (Figures 2J and 2K) with the lattice fringe spacing around 0.214 nm, corresponding to the (002) plane of Ru (Wang et al., 2020Wang Y. Zhu Q. Xie T. Peng Y. Liu S. Wang J. Promoted alkaline hydrogen evolution reaction performance of Ru/C by introducing TiO2 nanoparticle.ChemElectroChem. 2020; 7: 1182-1186Crossref Scopus (7) Google Scholar) (Figure 2L). Therefore, the average nanoparticle size decreased in the order of Pd > Rh > Rh2P > Ru among the investigated samples. Moreover, Rh2P/LC400 exhibits much smaller size of Rh nanoparticle with more homogeneous and more uniform dispersion if compared with Rh/LC400 (Figures 2A–2F). The observed porous architecture of Rh2P/LC400 should be favorable for mass transfer and diffusion in the investigated hydrogenative coupling reaction. The textural properties of the developed samples were investigated with N2 sorption isotherm (Figure 3A), Table 1 lists the resulting results. LC support exhibits a steep rise at low P/P0 zone (P/P0 < 0.1) with a very weak hysteresis loop from middle to high P/P0 zone (Figure 3A). Therefore, LC has a micropore-prevailing and hierarchically micro-mesoporous morphology (Deng et al., 2015Deng J. Xiong T. Xu F. Li M. Han C. Gong Y. Wang H. Wang Y. Inspired by bread leavening: one-pot synthesis of hierarchically porous carbon for supercapacitors.Green. Chem. 2015; 17: 4053-4060Crossref Google Scholar) (Figure S2) showing a specific Brunauer-Emmet-Teller (BET) surface area around 1,664 m2 g−1. After loading with transition metal, the resulting metallic catalyst exhibits a significantly reduced specific surface area (Table 1). For example, Rh2P/LC400 exhibits a specific surface area of 724 m2 g−1 (Table 1).Table 1Textural parameters of the investigated samplesSampleSBETaSBET, specific surface area. [m2 g−1]SmicrobSmicro, the specific surface area of micropore. [m2 g−1]SmesocSmeso, the specific surface area of mesopore. [m2 g−1]Dmicro/DmesodDmicro/Dmeso, the average diameters of micropore (Dmicro) and mesopore (Dmeso). [nm]VtotaleVtotal, the total specific pore volume. [cm3 g−1]Vmicro/VmesofVmicro/Vmeso, the specific pore volume of micropore (Vmicro) and mesopore (Vmeso). [cm3 g−1]LC166413203440.78/2.271.200.55/0.65Rh2P/LC4007241765481.02/4.020.450.08/0.37Rh2P/LC6005051783271.03/4.010.620.08/0.54Rh2P/LC8006962324641.01/4.000.710.11/0.60Rh/LC40011564846720.85/3.110.690.21/0.48Pd/LC4006301354950.85/3.080.410.12/0.29Ru/LC4005672742930.85/3.100.370.10/0.27recovered Rh2P/LC4004261952310.60/2.050.330.10/0.23recovered Rh/LC4007973254721.01/3.990.570.16/0.41a SBET, specific surface area.b Smicro, the specific surface area of micropore.c Smeso, the specific surface area of mesopore.d Dmicro/Dmeso, the average diameters of micropore (Dmicro) and mesopore (Dmeso).e Vtotal, the total specific pore volume.f Vmicro/Vmeso, the specific pore volume of micropore (Vmicro) and mesopore (Vmeso). Open table in a new tab All of the obtained samples were then performed with X-ray diffraction (XRD) analysis. Rh2P/LC400 shows a broad diffraction peak at 2θ = 21.0° (Figure 3B), which is indexed to the diffraction peak from amorphous carbon. In addition, five characteristic peaks at 2θ = 32.5, 46.8, 58.0, 68.2, and 77.8° are respectively assigned to the (200), (220), (222), (400), and (420) planes of Rh2P (JCPDF file no. 02-1299), suggesting the presence of Rh2P crystal on the LC surface (Duan et al., 2017Duan H. Li D. Tang Y. He Y. Ji S. Wang R. Lv H. Lopes P.P. Paulikas A.P. Li H. et al.High-performance Rh2P electrocatalyst for efficient water splitting.J. Am. Chem. Soc. 2017; 139: 5494-5502Crossref PubMed Scopus (240) Google Scholar). In the case of Rh/LC400, three representative diffraction peaks at 2θ values of 41.1, 47.8 and 69.9° (Figure 3B) are individually indexed to the (111), (200), and (220) planes of metallic Rh (JCPDF file no. 05-0685) (Kundu et al., 2018Kundu M.K. Mishra R. Bhowmik T. Barman S. Rhodium metal–rhodium oxide (Rh–Rh2O3) nanostructures with Pt-like or better activity towards hydrogen evolution and oxidation reactions (HER, HOR) in acid and base: correlating its HOR/HER activity with hydrogen binding energy and oxophilicity of the catalyst.J. Mater. Chem. A. 2018; 6: 23531-23541Google Scholar), which indicates successful loading of metallic Rh on the LC surface. Pd/LC400 displays three characteristic peaks at 2θ = 40.2, 46.7, and 68.2° (Figure 3B), which are respectively assigned to the (111), (200), and (220) planes of metallic Pd (JCPDF file no. 46-1043). Therefore, Pd(0) crystals, rather than palladium phosphide, are suggested to be deposited on the LC surface for the obtained Pd/LC400 (Li et al., 2016Li J. Tian Q. Jiang S. Zhang Y. Wu Y. Electrocatalytic performances of phosphorus doped carbon supported Pd towards formic acid oxidation.Electrochim. Acta. 2016; 213: 21-30Crossref Scopus (27) Google Scholar). Notably, Ru/LC400 only displays a weak characteristic peak at 2θ = 44.2° (Figure 3B), corresponding to the crystalline phases of Ru (JCPDF file no. 06-0063). This observation can presumably be attributed to the highly dispersed and homogeneous Ru species with small particle size on the LC surface (Zou et al., 2019Zou J. Wu M. Ning S. Huang L. Kang X. Chen S. [email protected] core-shell nanoparticles: impact of the atomic ordering of Ru metal core on the electrocatalytic activity of Pt shell.ACS Sustain. Chem. Eng. 2019; 7: 9007-9016Crossref Scopus (19) Google Scholar). Surface elements and their chemical states of the obtained samples were further examined by X-ray photoelectron spectroscopy (XPS). For Rh2P/LC400, the presence of Rh, P, C, and O elements is confirmed in the XPS survey (Figure 4A and Tables S3–S7). The high-resolution Rh 3d XPS of Rh2P/LC400 can be generally deconvolved into two sets of doublet peaks (Figure 4B). The set of strong doublet peaks, located at binding energies of 312.4 eV (indexed to Rh 3d3/2) and 307.7 eV (indexed to Rh 3d5/2), can be ascribed to metallic Rh, corresponding to Rh2P species (Luo et al., 2020Luo F. Guo L. Xie Y. Xu J. Cai W. Qu K. Yang Z. Robust hydrogen evolution reaction activity catalyzed by ultrasmall Rh-Rh2P nanoparticles.J. Mater. Chem. A. 2020; 8: 12378-12384Crossref Google Scholar). The other set of weak doublet peaks at 314.8 eV (indexed to Rh 3d3/2) and 309.8 eV (indexed to Rh 3d5/2) are ascribed to the Rh(III) oxidation state (Luo et al., 2020Luo F. Guo L. Xie Y. Xu J. Cai W. Qu K. Yang Z. Robust hydrogen evolution reaction activity catalyzed by ultrasmall Rh-Rh2P nanoparticles.J. Mater. Chem. A. 2020; 8: 12378-12384Crossref Google Scholar). Rh2P (73%) is formed as the predominant species on the Rh2P/LC400 surface based on the integration areas of these two doublets, accordingly demonstrating successful formation of Rh2P catalyst from Rh-dppe complex under the synthetic conditions. High-resolution P 2p3/2 XPS of Rh2P/LC400 (Figure 4C) can be deconvoluted into three characteristic peaks at the binding energy of 134.5 eV (indexed to P−O), 133.3 eV (indexed to P−C), and 130.1 eV (indexed to Rh−P). The presence of P−O species presumably originates from oxidation of the surface P upon exposure to air, whereas the existence of P−C indicates the formation of doped P atom into the LC matrix (Chen et al., 2016Chen Y.-Y. Zhang Y. Jiang W.-J. Zhang X. Dai Z. Wan L.-J. Hu J.-S. Pomegranate-like N,P-doped Mo2[email protected] nanospheres as highly active electrocatalysts for alkaline hydrogen evolution.ACS Nano. 2016; 10: 8851-8860Crossref PubMed Scopus (420) Google Scholar). Finally, the formation of Rh2P species can be demonstrated by the presence of Rh−P species from the P 2p3/2 XPS of Rh2P/LC400. Therefore, our XPS analysis of Rh2P/LC400 is in accordance with its TEM and XRD results, confirming the major Rh2P species on the Rh2P/LC400 surface. In the case of Rh/LC400, its high-resolution Rh 3d XPS can be deconvolved into two sets of doublet peaks (Figure 4B). The set of strong doublet peaks, located at binding energies of 312.3 eV (indexed to Rh 3d3/2) and 307.6 (indexed to Rh 3d5/2), can be ascribed to metallic Rh as a prevailing species (70%), whereas the set of weak doublet peaks at 314.7 eV (indexed to Rh 3d3/2) and 309.7eV (indexed to Rh 3d5/2) are ascribed to the Rh(III) oxides. Notably, Rh2P/LC400 shows evident shifts to higher binding energies in the Rh 3d XPS if compared with those of Rh/LC400 (Figure 4B). The observed positive shifts in the Rh 3d XPS indicate enhanced charge transfer to Rh as well as stronger interactions between P and Rh in the Rh2P/LC400 sample (Su et al., 2020Su J. Zhao H. Fu W. Tian W. Yang X. Zhang H. Ling F. Wang Y. Fine rhodium phosphides nanoparticles embedded in N, P dual-doped carbon film: new efficient electrocatalysts for ambient nitrogen fixation.Appl. Catal. B Environ. 2020; 265: 118589-118596Crossref Scopus (39) Google Scholar). The Rh-P interaction in Rh2P can modify the surface charge states and electron cloud density on the Rh site, which should be beneficial to H2 activation. For Pd/LC400 sample, its high-resolution Pd 3d XPS are deconvolved into two sets of doublet peaks (Figure 4D). The doublet peaks with strong intensity, located at a binding energy of 341.2 eV (indexed to Pd 3d3/2) and 336.0 eV (indexed to Pd 3d5/2), can be assigned to metallic Pd as a prevailing species (76%).