A Covalent Triazine Framework, Functionalized with Ir/N-Heterocyclic Carbene Sites, for the Efficient Hydrogenation of CO₂ to Formate

Abstract

Functionalizing the recently developed porous materials such as porous organic frameworks and coordination polymer networks with active homogeneous catalytic sites would offer new opportunities in the field of heterogeneous catalysis. In this regard, a novel covalent triazine framework functionalized with an Ir(III)-N-heterocyclic carbene complex was synthesized and characterized to have a coordination environment similar to that of its structurally related molecular Ir complex. Because of the strong σ-donating and poor π-accepting characters of the Nheterocyclic carbene (NHC) ligand, the heterogenized Ir-NHC complex efficiently catalyzes the hydrogenation of CO 2 to formate with a turnover frequency of up to 16 000 h −1 and a turnover number of up to 24 300; these are the highest values reported to date in heterogeneous catalysis for the hydrogenation of CO 2 to formate.

Covalent Triazine‐based Frameworks—Tailor‐made Catalysts and Catalyst Supports for Molecular and Nanoparticulate Species

“…In their very recent publication, Gunasekar et al enhanced CO 2 hydrogenation activity to formate by introducing cationic imidazolium units to the CTF support. [96] Since high CO 2 affinities have been observedf or these materials in prior studies, [51] they should display suitables upports for various CO 2 related applications. Impressive values for the initial TOF (up to 16 000 h À1 after 15 min) have been reported at maximum TONs of 24 300 after 15 ha t1 20 8Ca nd 80 bar equimolar CO 2 /H 2 for this solid molecular catalysts ystem.…”

Hydrogenation of CO2 to Formate over Ruthenium Immobilized on Solid Molecular Phosphines

“…Yoon and co‐workers reported [{IrCp*Cl 2 } 2 ] loaded onto a 2,2′‐bipyridine‐based covalent triazine framework (bpy‐CTF) and a heptazine‐based covalent organic framework, achieving TONs of 5000 and 6400, respectively . Recently, the same group proposed CTF functionalized with Ir−NHC sites (NHC=N‐heterocyclic carbene), which resulted in an enhanced TON of 24 300 after 15 h . This shows that the direct immobilization of molecular complexes onto functionalized porous polymers leads to improved catalytic activity and is a promising strategy for catalyst design .…”

Integration of an N‐Heterocyclic Carbene Precursor into a Covalent Triazine Framework for Organocatalysis

“…In previousw ork, imid-CTFs weres hown to capture CO 2 [20] and to hydrogenateC O 2 to formate by the aid of ac oordinated iridium catalyst. [21] However,t he best results were obtained by CTFs synthesized at 400 8Co ra bove. In contrast, thermogravimetric analysis revealed as ubstantial decomposition of the respective monomers startinga t3 00 8Ca lready.…”

“…[19] Thes uccessful integration of an imidazolium monomer as NHC precursor into a CTF would result in ar obust andf unctional catalyst, buti th as been rarely described. [20][21][22] In contrast to syntheses of MOFs [23][24][25] or EOFs, [26] an ionothermal trimerization of nitriles requirest emperatures of up to 400 8C. Therefore, chemically fragile monomers bearing aN HC precursor will not withstand these harsh conditions, and well-defined functionalities would simply decompose.…”

“…This issue should be taken into critical consideration when discussing the few examples of CTFs with embedded cationic imidazolium moieties (imid-CTF). [20][21][22] To generate the intermediary carbene species, ap roton has to be abstracted fromt he imidazolium cation precursors ystem prior to its targeted application. In previousw ork, imid-CTFs weres hown to capture CO 2 [20] and to hydrogenateC O 2 to formate by the aid of ac oordinated iridium catalyst.…”