Rational Preparation of Atomically Precise Non-Alkyl Tin-Oxo Clusters with Theoretical to Experimental Insights into Electrocatalytic CO 2 Reduction Applications

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Oct 2021Rational Preparation of Atomically Precise Non-Alkyl Tin-Oxo Clusters with Theoretical to Experimental Insights into Electrocatalytic CO2 Reduction Applications Di Wang†, Zhe-Ning Chen†, Qing-Rong Ding, Cheng-Cheng Feng, San-Tai Wang, Wei Zhuang and Lei Zhang Di Wang† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100049 †D. Wang and Z.-N. Chen contributed equally to the work.Google Scholar More articles by this author , Zhe-Ning Chen† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 †D. Wang and Z.-N. Chen contributed equally to the work.Google Scholar More articles by this author , Qing-Rong Ding State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Cheng-Cheng Feng State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , San-Tai Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Wei Zhuang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected]sm.ac.cn State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author and Lei Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000546 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Tin oxides (SnO2) have been widely utilized in electronics, nanolithography, and catalysis. As the atomically precise models of SnO2, tin-oxo clusters (TOCs) not only provide opportunities for mechanism studies, but they also extend potential applications through structural modulation. However, to date, most of the reported TOCs belong to alkyl organotin complexes, which are usually toxic to the human body and the active sites are difficult to expose for catalysis. In this work, we successfully developed a green synthesis directly using SnCl4 as precursors to prepare unprecedented non-alkyl TOCs. Unlike the former alkyl TOCs with Sn–C bond (inert), the surface of the obtained TOCs is completely functionalized by Cl ions and pyrazole ligands (active). Both density functional theory (DFT) calculations and electrocatalytic experiments indicated that the non-alkyl Sn10 cluster presented better CO2 reduction reaction (CO2RR) activity than the classical alkyl-Sn12 cluster. Moreover, to get further insights into the active center for CO2RR, an isostructural Ti-substituted cluster of Sn4Ti6 with only Sn–Cl bonds was prepared. The comparable electrocatalytic CO2RR activities between Sn10–(Cl, pyrazole) and Sn4Ti6–Cl confirmed the critical active roles of Sn–Cl sites. This work will benefit the future design of SnO2 materials with environmentally friendly applications in CO2 conversion. Download figure Download PowerPoint Introduction Nanosized metal oxides have been broadly used in both industry and laboratories as important functional materials with numerous applications.1–5 The surface modification of metal oxides is extremely crucial to their physical attributes.6 Considering the difficulty of identifying the interface structures of metal oxide nanoparticles, atomically precise metal-oxo clusters (MOCs) with well-defined composition and connectivity could provide ideal molecular models for structure-property relationship studies.7–10 Except for the common oxo bridges, MOCs usually present different terminal functionalization groups, which are highly dependent on the intrinsic characteristic of metal centers.11,12 Oxo clusters of transition metals (typically as polyoxometalates), for instance, easily present O-donor terminal groups but are difficult to functionalize by N-donor groups, not to mention by alkyl ligands.7,13,14 Meanwhile, oxo clusters of main group metals like Sn, Sb, Te, and others often show alkyl-type end groups.15–18 Such terminal functionalities have important influence on the properties of MOCs. It is quite appealing but also challenging to modify these terminal groups of MOCs, which would significantly change their electronic structures and reactivities. As a significant metal oxide material, tin oxide (SnO2) has been widely used in the fields of transparent electrodes, gas sensors, and solar cells.19–21 More recently, many research efforts have been devoted to developing SnO2-based electrocatalysts for CO2 reduction reactions (CO2RR), which promote the achievement of economically and environmentally important carbon-neutral energy cycles.22–24 In principle, tin-oxo clusters (TOCs) with precise interface structures could be used for experimental and theoretical understanding of CO2RR mechanisms at the molecular level. However, most of the reported TOCs belong to alkyl TOCs with tin-carbon bonds (Scheme 1), which are usually toxic to the human body and lead to serious environmental problems.16,25–33 Moreover, in the view of catalytic reactions, the tin-carbon bonding is rather stable, making it difficult to cleave to expose Sn atoms as active sites.34,35 If the terminal alkyl groups of TOCs could be replaced by easily dissociated inorganic ligands (e.g., halides), the obtained non-alkyl Sn–O clusters might be used as efficient model electrocatalysts for CO2RR to acquire mechanism information. Thus, functionalizing TOCs with non-alkyl ligands is extremely important, which will enrich their structural chemistry, increase their environmental friendliness, and promote their catalytic applications. Unfortunately, although some organometallic tin complexes without Sn–C bonds have been prepared by Jurkschat and co-workers,36–38 the field of high nuclearity non-alkyl TOCs still remains to be developed. It is especially urgent to explore a green and facile synthetic approach to non-alkyl TOCs from low-toxic inorganic tin precursors. Scheme 1 | Comparison between the previously reported alkyl TOCs and herein developed non-alkyl TOCs, highlighting the stable C–Sn and reactive Cl–Sn terminal bonding. Download figure Download PowerPoint In this work, pyrazole-thermal synthesis, where pyrazole simultaneously acts as the hydrolysis delay ligand and reaction medium, has been introduced to the field of TOCs for the first time. Through using SnCl4·5H2O as an inorganic tin precursor, two unprecedented non-alkyl TOCs, Sn14(μ3-O)12(μ2-O)4(μ2-OH)4(PZ)8(HPZ)4Cl12 (PZ = pyrazole) ( TOC-21) and Sn10(μ3-O)8(μ2-O)4(PZ)8(HPZ)4Cl8 ( TOC-22) were successfully prepared. Single-crystal X-ray diffraction (SXRD) analysis revealed that both the Sn14 core of TOC-21 and the Sn10 core of TOC-22 were completely stabilized by pyrazole ligands and Cl− ions. Density functional theory (DFT) calculations were applied to study the probable paths for activation of TOCs and their potential performance in electrocatalytic CO2RR. The DFT results showed that the non-alkyl cluster TOC-22 exhibited much better feasibility, both in cluster charging and in ligand detachment. Meanwhile, this cluster possessed a lower overpotential in CO2RR than the typical alkyl-Sn12 cluster, suggesting a greater potential for non-alkyl TOC in catalysis of CO2RR. Accordingly, in experimental electrocatalytic studies, the application of non-alkyl Sn10 clusters selectively converted CO2 into formate with the highest faradaic efficiency (FE) of ∼57% at −1.0 V (vs reversible hydrogen electrode [RHE]), which is indeed much higher than that of alkyl-Sn12 [∼26% at −1.2 V (vs RHE)]. Moreover, to get further insights into the active center on the non-alkyl Sn10 cluster, a Ti-substituted isostructural heterometallic cluster of [Sn4Ti6(μ3-O)8(μ2-O)4(PZ)8(HPZ)4Cl8] ( TOC-23) with only Cl− ions as terminal Sn-functionality was constructed. Our investigation revealed comparable activity between TOC-22 and TOC-23 clusters, suggesting Sn–Cl sites as the critical active centers in the electrocatalytic CO2RR. Experimental Section Materials, instruments, and methods All the chemical reagents were commercially purchased and used without further purification. SnCl4·5H2O, imidazole, pyrazole, Ti(OiPr)4, and butyltin oxide were purchased from Aladdin (Shanghai, China). The infrared (IR) spectrum was obtained on a VERTEX 70v spectrometer with pressed KBr pellets in the range of 4000–400 cm−1. Powder XRD patterns were obtained by using a Miniflex diffractometer with CuKα radiation (λ = 1.54056 Å). Elemental analyses (C, H, and N) were performed on a Vario MICRO elemental analyzer. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/sequential differential thermal analyses (SDTA) 851e thermal analyzer in flowing N2 atmosphere with a heating rate of 10 °C/min. The proton nuclear magnetic resonance (1H NMR) experiments were carried out on a JNM-ECZ400S spectrometer at a frequency of 400 MHz. The liquid products were detected by a CIC-D100 automatic range ion chromatograph. Gas chromatography (GC) was performed with a GC-2014C (Shimadzu, Shanghai, China) GC system equipped with flame-ionization detectors and a thermal conductivity detector (TCD). Crystallographic data for this paper have been deposited in the Cambridge Crystallographic Data Centre as CCDC 2017923-2017925 for TOC-21 to TOC-23. Details of SXRD analysis, DFT computational studies, and electrochemical measurements are presented in the Supporting Information. Synthesis of TOC-21 SnCl4·5H2O (0.35 g, 1.0 mmol) and pyrazole (2.0 g, 29.38 mmol) were mixed and sealed in a 20 mL vial, and then transferred to a preheated oven at 100 °C and heated for 4 days. After cooling to room temperature, colorless crystals of TOC-21 were obtained with a yield of ∼25% based on Sn. Elemental analysis (%): Calcd for C48H56Cl12N32O20Sn14: C, 16.53; H, 1.62; N, 12.20. Found: C, 17.56; H, 1.89; N, 12.69. Synthesis of TOC-22 SnCl4·5H2O (0.35 g, 1.0 mmol), pyrazole (2.0 g, 29.38 mmol), imidazole (0.20 g, 2.94 mmol), or benzimidazole (0.15 g, 1.27 mmol) were mixed and sealed in a 20 mL vial, and then transferred to a preheated oven at 100 °C and heated for 4 days. After cooling to room temperature, colorless crystals of TOC-22 were obtained with a yield of ∼47% based on Sn. Elemental analysis (%): Calcd for C36H36Cl8N24O12Sn10: C, 17.52; H, 1.47; N, 13.62. Found: C, 19.20; H, 1.84; N, 14.90. Synthesis of TOC-23 SnCl4·5H2O (0.35 g, 1.0 mmol) and pyrazole (2.0 g, 29.38 mmol) were mixed together. Then Ti(OiPr)4 (0.50 ml, 1.50 mmol) was added. The mixture was then sealed in a 20 mL vial and transferred to an oven preheated to 100 °C for 4 days. After cooling to room temperature, yellow crystals of TOC-23 were obtained with a yield of ∼16% based on Sn. Elemental analysis (%): Calcd for C42H51Cl8N28O12Sn4Ti6: C, 23.08; H, 2.35; N, 17.94. Found: C, 22.59; H, 2.14; N, 17.42. Results and Discussion Green synthesis of non-alkyl TOCs by coordination delayed hydrolysis of SnCl4 Generally, the synthetic methods determine the structures and properties of materials. As the most majority of tin-oxo clusters, alkyl TOCs are assembled from toxic organotin precursors. Even for the few aminotin compounds without C–Sn bonding, organotin reactants and harsh organometallic conditions are usually essential.39,40 Naturally, for the environmentally friendly preparation of non-alkyl TOCs, the most intuitive method would be to directly use inorganic tin salts like SnCl4 as the starting materials. However, due to its highly hydrolysable characteristics, uncontrolled hydrolysis arises easily to form heterogeneous SnO2 nanoparticles under conventional reaction conditions ( Supporting Information Scheme S1). To slow the hydrolysis of Sn4+ to give rise to crystalline TOCs, proper protecting ligands are needed to separate it from excessive H2O. Based on these considerations, moderate Sn coordinating pyrazoles are selected as hydrolysis delay ligands. Meanwhile, the low melting point of pyrazoles (∼66−70 °C) enables them to act as possible reaction mediums at high temperature (referred to as pyrazole-thermal synthesis).41 Interestingly, nontoxic inorganic tin salts can be applied in such pyrazole-thermal synthesis, making it a real green synthesis. The solvated pyrazoles could surround the Sn4+ ions to prevent them from fast and disordered hydrolysis. And the reserved pyrazole ligands will also stabilize the produced TOC cores and promote their crystallization for structural analysis. Atomic structures of the obtained non-alkyl TOCs Following the above synthetic strategy, the coordination delayed hydrolysis of SnCl4·5H2O was first carried out in pyrazoles at 100 °C, which gave rise to well-crystallized products of TOC-21. SXRD analysis showed that TOC-21 crystallized in the tetragonal space group P-4. The cluster core of TOC-21 was composed of 14 Sn atoms connected by 4 μ2-O, 4 μ2-OH, and 12 μ3-O (Figures 1a and 1b). Interestingly, the Sn14 core of TOC-21 was completely stabilized by 4 monocoordination pyrazoles, 8 bridged pyrazoles, and 12 Cl terminal ligands. As shown in Figure 1c, there were four different six-coordinated modes of Sn in TOC-21, [SnO2N2Cl2], [SnO4NCl], [SnO4N2], and [SnO5N], which were only constructed by O, N, and Cl atoms. There was no C–Sn bonding in the structure of TOC-21, making it a real non-alkyl TOC. Figure 1 | (a) Molecular structure of TOC-21. (b) The Sn14O20 core of TOC-21, highlighting the 12 terminal Cl− ligands. (c) The coordination spheres of the Sn atoms in TOC-21. H atoms are omitted for clarity. Sn, purple; O, red; N, blue; C, gray; Cl, cyan. Download figure Download PowerPoint The successful construction of TOC-21 confirmed the feasibility of the coordination delayed hydrolysis method. Actually, if different delayed ligands with diverse coordination behaviors were introduced, this strategy might also provide a tool to modulate the structures of obtained clusters. Indeed, when a small amount of imidazole with the same composition as pyrazole but a different configuration, was introduced to the reactions, crystals of a new TOC of TOC-22 were obtained in high yield. SXRD analysis showed that the Sn10 core of TOC-22 was also completely stabilized by non-alkyl ligands, including four monocoordinated pyrazoles, eight bridged pyrazoles, and eight Cl terminal ligands (Figure 2a). To understand the structure of TOC-22 more clearly, it can be regarded as two parts, the inside {Sn6} octahedron and the four peripheral Sn atoms, which are connected through four μ3-O (Figure 2b). The Sn atoms in these two parts display different coordination models as the [SnO4N2] for central {Sn6} octahedron and [SnO2N2Cl2] for the peripheral Sn, respectively (Figure 2c). Figure 2 | (a) Molecular structure of TOC-22. (b) The Sn10O10 core of TOC-22, highlighting the central Sn6 octahedron and the eight terminal Cl− ligands. (c) The coordination spheres of the Sn atoms in TOC-22. (d) Molecular structure of TOC-23. (e) The Sn4Ti6O10 core of TOC-23, highlighting the central Ti6 octahedron and the eight terminal Cl− ligands. (f) The coordination spheres of the Sn and Ti atoms in TOC-23. H atoms are omitted for clarity. Ti, green. Download figure Download PowerPoint Interestingly, the developed synthetic methodology is also effective to prepare heterometallic non-alkyl TOCs. By simply introducing Ti(OiPr)4 into the reaction of TOC-21, the bimetallic Sn4Ti6-oxo clusters of TOC-23 were successfully obtained. SXRD structural analysis indicated that the cluster topology of TOC-23 was almost identical to TOC-22, except that the central {Sn6} octahedron was replaced by a {Ti6} octahedron (Figures 2d and 2e). Meanwhile, the monocoordinated pyrazole functionalization on Sn disappeared, and only Cl terminal ligands were retained on the peripheral Sn atoms to keep the [SnO2N2Cl2] coordination sphere (Figure 2f). The Sn–Cl bond distances in the obtained compounds were between 2.341 and 2.441 Å, which were comparable with reported values.42,43 Theoretical and experimental insights into the applications in electrocatalytic CO2 reduction The functionalization of TOCs by different ligands greatly affects their electronic structures and reactivities.44,45 Especially, for catalytic reactions, the interface structures of catalysts directly determine their activities.46–49 The above prepared non-alkyl TOCs provided ideal platforms to clarify this influence. As shown in Figures 3a and 3b, the Sn10 core of TOC-22 and the Sn4Ti6 core of TOC-23 were both surrounded by pyrazoles and Cl. In the representative alkyltin cluster, [Na(BuSn12)(μ4-O)4(μ2-O)5(μ-OH)7(CH3O)12] ( Alkyl-Sn12),30 the interface consisted of Sn-alky bonds (Figure 3c). Moreover, the successful construction of TOC-23 provided an ideal model for investigating the influence of metal substitution and coordination environment revision on the catalytic properties of non-alkyl TOCs. Thereupon, comparative DFT calculations were then carried out on TOC-22, TOC-23, and Alkyl-Sn12 to illustrate their potential reactivities as catalysts. Figure 3 | (a–c) Space-filling modes of Sn10, Sn4Ti6, and Sn12 cores of TOC-22, TOC-23, and Alkyl-Sn12, respectively, highlighting the functionalized Cl, pyrazole, and alkyl groups on the cluster surface. Download figure Download PowerPoint Because all the currently synthesized TOCs possess the saturated structures, we reasoned that the ligand detachment from the Sn center should be the initial step to generate the active catalytic species. At the start, the ligand dissociation free energies for the models of non-alkyl-Sn10 cluster TOC-22, non-alkyl-Sn4Ti6 cluster TOC-23, and alkyl-Sn12 cluster were calculated to reveal their potential to generate the active catalytic species. As shown in Figure 4, the chloride ion, η1-pyrazole (η1-C3H4N2), and η2-pyrazole (η2-C3H3N2−) on non-alkyl clusters, as well as alkyl (C2H5−) and methoxy (OCH3−) on alkyl cluster, were all considered as possible dissociated ligands. Our DFT calculations showed that the direct ligand detachment from the neutral TOCs was not feasible. Instead, the clusters could be charged at first and then experience ligand dissociation to generate the active catalytic species. As displayed in Figure 4a, the charging of the non-alkyl-Sn10 cluster of TOC-22 is feasible with a small free-energy change of 4.0 kcal/mol. Subsequently, the dissociation of the chloride (Sn1 site) and η1-pyrazole (Sn2 site) ligands occurred with the moderate free-energy change of 12.0 and 16.5 kcal/mol, respectively. However, a strong binding strength for the η2-pyrazole ligand on the charged TOC-22 was observed by DFT calculations, with the dissociation free energy as high as 45.6 kcal/mol. These findings indicated the potential for dissociation of the chloride and η1-pyrazole ligands to generate the active catalytic species. The possibility for η2-pyrazole ligand detachment was excluded. Notably, in comparison with the alkyl cluster, the non-alkyl cluster TOC-22 exhibited a much better feasibility both in cluster charging as well as ligand detachment (Figure 4b), suggesting greater potential for activation of non-alkyl clusters to generate active catalytic species. Figure 4 | DFT calculated free-energy changes (kcal/mol) for the ligand detachment from (a) non-alkyl-Sn10 cluster TOC-22 and non-alkyl-Sn4Ti6 cluster TOC-23 (in parenthesis) as well as (b) Alkyl-Sn12 cluster. Download figure Download PowerPoint The reactivity of the resultant charged unsaturated intermediates was further directly evaluated by considering the probable electrochemical processes of CO2RR to formate (HCOO−). A pathway via *OCHO intermediate was proposed since it is believed that *OCHO is the key intermediate for the CO2RR to HCOO− transformation.24 Here, we considered a two-electron reaction path for CO2RR to HCOO−. First, *OCHO intermediate is formed through a proton coupled electron transfer to CO2. Then, this resultant *OCHO intermediate would be further reduced by adding one more electron to produce the HCOO−. As shown in Figure 5, the free-energy diagram was calculated at U = −0.44 V versus RHE, according to the calculated standard reduction potential of the overall reaction of CO2 + H+ + 2e → HCOO −. Our DFT calculations showed that the Sn1 (Sn–Cl) was the most active site in TOC-22, in which the calculated overpotential for CO2RR was −0.81 V, whereas reaction on Sn2 site (Sn–η1–C3H4N2) exhibited a higher overpotential of −1.10 V. These findings demonstrate the significant role of detachable chlorine ligand in the catalytic activity of TOC-22. We were intrigued to find that the non-alkyl-Sn4Ti6 cluster TOC-23, in which all the Sn1 sites were retained, still held the reactivity in CO2RR. As shown in Figure 5, compared with the pure TOC TOC-22, TOC-23 exhibited higher free-energy change for chloride ligand detachment as well as the overpotential for CO2RR and suggested a relatively lower reactivity. However, it is worth noting that the charging of TOC-23 was more favorable than TOC-22, implying better catalytic performance for TOC-23 in a low potential. Figure 5 | Free-energy diagram for CO2RR to HCOO− on the active catalytic species at U = −0.44 V versus RHE. Numbers in parenthesis are calculated overpotentials. Download figure Download PowerPoint A good catalytic system needs to balance stability and activity. In the present system, that balance is related to the ligand detachment from its cluster. A “stable” cluster often possesses “stable” ligand coordination, and therefore exhibits poor catalytic activity, such as the common alkyl clusters. Our currently prepared non-alkyl clusters provide the opportunity to achieve the desired balance between stability and activity. We have found that the detachable chlorine ligand is closely related to the observed catalytic activity. In addition, we suggest that the pyrazole ligand with unoccupied π orbitals is related to the cluster charging. Intriguingly, as compared with the pure non-alkyl-Sn10 cluster TOC-22, the charging of heterometallic non-alkyl-Sn4Ti6 cluster TOC-23 is easier, which can be attributed to the contribution from the unoccupied d orbitals of Ti4+. According to the results, two points can be summarized for designing more efficient non-alkyl cluster catalysts: (1) introducing a proper detachable ligand in the non-alkyl clusters is key for the catalytic activity; (2) facilitating the cluster charging by proper ligand as well as transition metal incorporating can further promote the catalytic activity. To further verify the above theoretical calculation results, experimental investigations were then carried out to check the electrocatalytic performance of TOC-22, TOC-23, and Alkyl-Sn12on CO2RR. To this end, the carbon papers decorated by TOC-22, TOC-23, and Alkyl-Sn12 were used as working electrodes for CO2RR, respectively. Linear sweep voltammetry (LSV) profiles were first measured in Ar or CO2 saturated 0.5M KHCO3 solution, and the data were collected from 0 to −1.2 V (vs RHE) with a scan rate of 5 mV/s. It can be clearly seen that the current density of the three electrodes in CO2 saturated electrolyte was higher than those in Ar-saturated electrolyte, indicating that all of them underwent CO2RR (Figure 6a). In addition, the onset potentials of TOC-22 and TOC-23 were both lower than that of Alky-Sn12 in the CO2 saturated KHCO3 solution, indicating faster kinetics of electrocatalytic CO2RR by non-alkyl TOCs than Alky-Sn12. And TOC-22 decorated electrode showed the highest current density in the tested electrocatalysts. Powder XRD analysis further confirmed that the samples of TOC-22, TOC-23, and Alky-Sn12 were relatively stable after soaking in 0.5M KHCO3 electrolyte for 12 h, which also remained rather intact after electrolysis ( Supporting Information Figures S3–S5). Moreover, the current density fluctuated little during the catalytic process, further revealing relatively good electrocatalytic durability ( Supporting Information Figures S18–S20). To identify the reduction products, GC and NMR spectroscopy were applied to analyze the gas- and liquid-phase products during the CO2RR process. The results indicated that formate was the only liquid CO2RR product, and a small amount of CO was detected in the gas phase (Figures 6b and Supporting Information Figures S24–S26). Meanwhile, some H2 was also produced by competing reaction, which strongly depends upon the applied potential ( Supporting Information Figures S27–S29). Figure 6 | (a) LSV of TOC-21, TOC-22, and Alkyl-Sn12 in Ar or CO2 saturated 0.5 M KHCO3 solution. (b) 1H NMR spectrum of the KHCO3 catholyte after 1200 s of CO2 reduction on TOC-21, TOC-22, and Alkyl-Sn12-derived electrodes, E(RHE) = −1.0 V. (c and d) The comparative FE of formate for TOC-21, TOC-22, and Alkyl-Sn12-derived electrodes at various potentials. Download figure Download PowerPoint Ion chromatograph analysis was then used to better quantify the produced formates. As shown in Figures 6c and 6d, both the FEformate of the non-alkyl clusters TOC-22 and TOC-23 were obviously higher than those of Alkyl-Sn12, which was in good agreement with the above theoretical analysis. Thereinto, the highest FEformate of TOC-22 and TOC-23 reached about 57.08% and 49.56%, respectively, at −1.0 V (vs RHE); while the best FEformate of Alkyl-Sn12 was only 26.34% at −1.2 V (vs RHE). It is also worth noting that at most applied potentials, the FEformate of Sn10–(Cl, pyrazole) cluster in TOC-22 and Sn4Ti6–Cl cluster in TOC-23 were quite close to each other, further confirming that the Sn–Cl sites are the main active sites for electrocatalytic CO2RR. Relatively higher FEformate value was observed for TOC-23 than TOC-22 at the low potential of −0.7 V (vs RHE), which might be due to the easier charging of heterometallic Sn4Ti6 clusters as illustrated by DFT calculations. Therefore, these experimental results are significantly consistent with the above theoretical studies, both of which confirm the better CO2RR activity of the prepared non-alkyl TOCs than the traditional alkyl ones. Conclusion A series of unprecedented non-alkyl TOCs have been successfully prepared directly using nontoxic SnCl4 as the precursor by pyrazole-thermal synthesis. Coordination delayed hydrolysis strategy played an important role during the cluster assembly, where pyrazoles simultaneously acted as the hydrolysis delay ligands and as the reaction medium. Through introducing different delaying ligands or heterometals, Sn14, Sn10, and Sn4Ti6-oxo clusters have been obtained. XRD structural analysis confirms that these Sn-oxo cores are completely functionalized by pyrazole ligands and Cl ions, and no C–Sn bonding as traditional alkyltin clusters is presented, making them real non-alkyl TOCs. Both theoretical calculations and experimental studies indicate that such modification on the functionalizing groups of clusters has significant influence on the electronic structures and catalytic activities. Compared with the C–Sn bonding in alkyltin clusters, the ligands on non-alkyl TOCs, especially the Cl ions, are much easier to dissociate to expose active sites for CO2RR. Accordingly, obviously higher formates production has been observed by both non-alkyl Sn10 and Sn4Ti6 clusters than the typical alkyl-Sn12 cluster. The success of this work represents a milestone in the rational construction of environmentally friendly non-alkyl TOCs and provides an effective method for modulating their physical properties. Meanwhile, the strategy developed can be further applied to other MOCs, not only enriching their structural chemistry but also extending their potential applications. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no conflicts of interests. Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 91961108, 21922111, a