Electrocatalytic reduction of CO2 using the dinuclear rhenium(I) complex [ReCl(CO)3(μ-tptzH)Re(CO)3]

Functionalized Triazines and Tetrazines: Synthesis and Applications

The molecules possessing triazine and tetrazine moieties belong to a special class of heterocyclic compounds. Both triazines and tetrazines are building blocks and have provided a new dimension to the design of biologically important organic molecules. Several of their derivatives with fine-tuned electronic properties have been identified as multifunctional, adaptable, switchable, remarkably antifungal, anticancer, antiviral, antitumor, cardiotonic, anti-HIV, analgesic, anti-protozoal, etc. The objective of this review is to comprehensively describe the recent developments in synthesis, coordination properties, and various applications of triazine and tetrazine molecules. The rich literature demonstrates various synthetic routes for a variety of triazines and tetrazines through microwave-assisted, solid-phase, metal-based, [4+2] cycloaddition, and multicomponent one-pot reactions. Synthetic approaches contain linear, angular, and fused triazine and tetrazine heterocycles through a combinatorial method. Notably, the triazines and tetrazines undergo a variety of organic transformations, including electrophilic addition, coupling, nucleophilic displacement, and intramolecular cyclization. The mechanistic aspects of these heterocycles are discussed in a detailed way. The bioorthogonal application of these polyazines with various strained alkenes and alkynes provides a new prospect for investigations in chemical biology. This review systematically encapsulates the recent developments and challenges in the synthesis and possible potential applications of various triazine and tetrazine systems.

Solution Studies and Crystal Structures of Heteropolynuclear Potassium/Copper Complexes with Phytate and Aromatic Polyamines: Self-Assembly through Coordinative and Supramolecular Interactions

Phytate (L^12- ) is a relevant natural product. It interacts strongly with biologically relevant cations, due to the high negative charge exhibited in a wide pH range. The synthesis and crystal structures of the mixed-ligand Cu(II) polynuclear complexes K(H₂ tptz)_0.5 [Cu(H₈ L)(tptz)] ⋅ 3.6H₂ O (1), K(H₂ O)₃ {[Cu(H₂ O)(bpca)]₃ (H₈ L)} ⋅ 1.75H₂ O (2), and K_1.5 (H₂ O)₂ [Cu(bpca)](H_9.5 L) ⋅ 8H₂ O (3) (tptz=2,4,6-tri(pyridin-2-yl)-1,3,5-triazine; Hbpca=bis(2-pyridylcarbonyl) amine) are reported herein. They were obtained by the use of an aromatic rigid amine, which satisfies some of the metal coordination sites and promotes the hierarchical assembly of 2D polymeric structures. Speciation of phytate-Cu(II)-Hbpca system and determination of complex stability constants were performed by means of potentiometric titrations, in 0.15 M NMe₄ Cl at 37.0 °C, showing that, even in solution, this system is able to produce highly aggregated complexes, such as [Cu₃ (bpca)₃ (H₇ L)]^2- . Furthermore, the Cu(II)-mediated tptz hydrolysis was studied by UV-vis spectroscopy at room temperature in 0.15 M NMe₄ Cl. Based on the equilibrium results and with the aid of molecular modelling tools, a plausible self-assembly process for 2 and 3 could be proposed.

Electrocatalytic CO2 Reduction with Cis and Trans Conformers of a Rigid Dinuclear Rhenium Complex: Comparing the Monometallic and Cooperative Bimetallic Pathways

Anthracene-bridged dinuclear rhenium complexes are reported for electrocatalytic carbon dioxide (CO₂) reduction to carbon monoxide (CO). Related by hindered rotation of each rhenium active site to either side of the anthracene bridge, cis and trans conformers have been isolated and characterized. Electrochemical studies reveal distinct mechanisms, whereby the cis conformer operates via cooperative bimetallic CO₂ activation and conversion and the trans conformer reduces CO₂ through well-established single-site and bimolecular pathways analogous to Re(bpy)(CO)₃Cl. Higher turnover frequencies are observed for the cis conformer (35.3 s^-1) relative to the trans conformer (22.9 s^-1), with both outperforming Re(bpy)(CO)₃Cl (11.1 s^-1). Notably, at low applied potentials, the cis conformer does not catalyze the reductive disproportionation of CO₂ to CO and CO₃^2- in contrast to the trans conformer and mononuclear catalyst, demonstrating that the orientation of active sites and structure of the dinuclear cis complex dictate an alternative catalytic pathway. Further, UV-vis spectroelectrochemical experiments demonstrate that the anthracene bridge prevents intramolecular formation of a deactivated Re-Re-bonded dimer. Indeed, the cis conformer also avoids intermolecular Re-Re bond formation.

Dinuclear Rhenium Complex with a Proton Responsive Ligand as a Redox Catalyst for the Electrochemical CO2 Reduction

Herein, we present the reduction chemistry of a dinuclear α-diimine rhenium complex, 1, [Re₂(L)(CO)₆Cl₂], with a proton responsive ligand and its application as a catalyst in the electrochemical CO₂ reduction reaction (L = 4-tert-butyl-2,6-bis(6-(1H-imidazol-2-yl)-pyridin-2-yl)phenol). The complex has a phenol group in close proximity to the active center, which may act as a proton relay during catalysis, and pyridine-NH-imidazole units as α-diimine donors. The complex is an active catalyst for the electrochemical CO₂ reduction reaction. CO is the main product after catalysis, and only small amounts of H₂ were observed, which can be related to the ligand reactivity. The i_c/i_p ratio of 20 in dimethylformamide (DMF) + 10% water for 1 points to a higher activity with regard to [Re(bpy)(CO)₃Cl] in MeCN/H₂O, albeit 1 requires a slightly larger overpotential (bpy = 2,2'-bipyridine). Spectroscopic and theoretical investigations revealed detailed information about the reduction chemistry of 1. The complex exhibits two reduction processes in DMF, and each process was identified as a two-electron reduction in the absence of CO₂. The first 2e^- reduction is ligand based and leads to homolytic N-H bond cleavage reactions at the imidazole units of 1, which is equal to a net double proton removal from 1 forming [Re₂(LH_-2)(CO)₆Cl₂]^2-. The second 2e^- reduction process has been identified as an O-H bond cleavage reaction at the phenol group, removal of chloride ions from the coordination spheres of the metal ions, and a ligand-centered one-electron reduction of [Re₂(LH_-3)(CO)₆Cl]^2-. In the presence of CO₂, the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO₂ instead of O-H bond cleavage.