Artificial receptors inspired by metalloenzymes share three key properties: a structurally flexible cavity, substrate binding via metal–ligand coordination, and metal-based redox activity. Herein, we report an organometallic nanotube with such features based on our supramolecular pillarplex platform, with eight CuI centers integrated in its cavitand walls. The structurally adaptable cavity of this recep-tor enables the endohedral coordination of tetrahydrofuran (THF) as a hydrophilic model substrate with remarkable binding affinity de-spite a steric mismatch between the host and guest. Evidence from SC-XRD, 1H NMR titration in aqueous solution, and DFT modelling confirms that metal–ligand coordination governs substrate binding. Electrochemical analysis of a derived rotaxane reveals metal-centered redox activity.
A new pathway of electrocatalytic transfer hydrogenation with neutral water as the H-donor was discovered using [(tBuPCP)Ir(H)(Cl)] (1) as the catalyst and styrene as a model substrate in THF electrolyte. Cyclic voltammetry experiments with 1 revealed that two subsequent reductions at –2.55 and –2.84 V vs. Fc+/Fc trigger the elimination of Cl– and afford the highly reactive anionic Ir(I) hydride complex [(tBuPCP)Ir(H)]– (2). The identity of 2 and its reactivity were further investigated by LIFDI-MS, which confirmed 2 as reactive species in the alkene hydrogenation cycle. Bulk electrolysis and chronoamperometry for electro-hydrogenation of styrene established ethylbenzene as the only product, formed with high faradaic efficiency of 96% and a turnover frequency of 1670 h–1 at an electrolysis potential of –3.1 V, with insignificant H2 formation. Importantly, the electro-hydrogenation performance of 1 remained constant upon addition of KOH to the electrolyte, which suggests a reaction mechanism that is independent of free H+. Instead, the reactive Ir-hydrides are regenerated by oxidative addition of H2O to the complex, which creates a reaction cascade that is reminiscent of metal-hydride formation in classical transfer hydrogenation systems. As such, the herein presented study on electrocatalytic transfer hydrogenation (e-TH) with H2O as the H-donor is different from the plethora of other electro-hydrogenation studies that operate via H+ reduction, often in low-pH electrolyte. These findings may inspire the general, pH independent use of H2O as H-donor in conjunction with electrochemistry, to replace isopropanol or formate as intrinsically reducing H-donors in the many existing examples of classical transfer hydrogenation.
With the aim to design water-soluble organometallic Ru(II) complexes acting as anticancer agents catalyzing transfer hydrogenation (TH) reactions with biomolecules, we have synthesized four Ru(II) monocarbonyl complexes (1–4), featuring the 1,4-bis(diphenylphosphino)butane (dppb) ligand and different bidentate nitrogen (N∧N) ligands, of general formula [Ru(OAc)CO(dppb)(N∧N)]n (n = +1, 0; OAc = acetate). The compounds have been characterized by different methods, including 1H and 31P NMR spectroscopies, electrochemistry, as well as single-crystal X-ray diffraction in the case of 1 and 4. The compounds have also been studied for their hydrolysis in an aqueous environment and for the catalytic regioselective reduction of nicotinamide adenine dinucleotide (NAD+) to 1,4-dihydronicotinamide adenine dinucleotide (1,4-NADH) in aqueous solution with sodium formate as a hydride source. Moreover, the stoichiometric and catalytic oxidation of 1,4-NADH have also been investigated by UV–visible spectrophotometry and NMR spectroscopy. The results suggest that the catalytic cycle can start directly from the intact Ru(II) compound or from its aquo/hydroxo species (in the case of 1–3) to afford the hydride ruthenium complex. Overall, initial structure–activity relationships could be inferred which point toward the influence of the extension of the aromatic N∧N ligand in the cationic complexes 1–3 on TH in both reduction/oxidation processes. While complex 3 is the most active in TH from NADH to O2, the neutral complex 4, featuring a picolinamidate N∧N ligand, stands out as the most active catalyst for the reduction of NAD+, while being completely inactive toward NADH oxidation. The compound can also convert pyruvate into lactate in the presence of formate, albeit with scarce efficiency. In any case, for all compounds, Ru(II) hydride intermediates could be observed and even isolated in the case of complexes 1–3. Together, insights from the kinetic and electrochemical characterization suggest that, in the case of Ru(II) complexes 1–3, catalytic NADH oxidation sees the H-transfer from 1,4-NADH as the rate-limiting step, whereas for NAD+ hydrogenation with formate as the H-donor, the rate-limiting step is the transfer of the ruthenium hydride to the NAD+ substrate, as also suggested by density functional theory (DFT) calculations. Compound 4, stable with respect to hydrolysis in aqueous solution, appears to operate via a different mechanism with respect to the other derivatives. Finally, the anticancer activity and ability to form reactive oxygen species (ROS) of complexes 1–3 have been studied in cancerous and nontumorigenic cells in vitro. Noteworthy, the conversion of aldehydes to alcohols could be achieved by the three Ru(II) catalysts in living cells, as assessed by fluorescence microscopy. Furthermore, the formation of Ru(II) hydride intermediate upon treatment of cancer cell extracts with complex 3 has been detected by 1H NMR spectroscopy. Overall, this study paves the way to the application of non-arene-based organometallic complexes as TH catalysts in a biological environment.
Abstract Electrocatalytic hydrogenation of 1‐octene as non‐activated model substrate with neutral water as H‐donor is reported, using [( t Bu PCP)Ir(H)(Cl)] ( 1 ) as the catalyst, to form octane with high faradaic efficiency (FE) of 96 % and a k obs of 87 s −1 . Cyclic voltammetry with 1 revealed that two subsequent reductions trigger the elimination of Cl − and afford the highly reactive anionic Ir(I) hydride complex [( t Bu PCP)Ir(H)] − ( 2 ), a previously merely proposed intermediate for which we now report first experimental data by mass spectrometry. In absence of alkene, the stoichiometric electrolysis of 1 in THF with water selectively affords the Ir(III) dihydride complex [( t Bu PCP)Ir(H) 2 ] ( 3 ) in 88 % FE from the reaction of 2 with H 2 O. Complex 3 then hydrogenates the alkene in classical fashion. The presented electro‐hydrogenation works with extremely high FE, because the iridium hydrides are water stable, which prevents H 2 formation. Even in strongly alkaline conditions (Bu 4 NOH added), the electro‐hydrogenation of 1‐octene with 1 also proceeds cleanly (89 % FE), suggesting a highly robust process that may rely on H 2 O activation, reminiscent to transfer hydrogenation pathways, instead of classical H + reduction. DFT calculations confirmed oxidative addition of H 2 O as a key step in this context.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
With the aim to design new water-soluble organometallic Ru(II) complexes acting as anticancer agents catalysing transfer hydrogenation (TH) reactions with biomolecules, we have synthesized four Ru(II) monocarbonyl complexes (1-4) featuring the 1,4-bis(diphenylphosphino)butane (dppb) ligand and different bidentate nitrogen (N^N) ligands, of general formula [Ru(OAc)CO(dppb)(N^N)]n (n = +1, 0; OAc = acetate). The compounds have been characterised by different methods, including 1H and 31P NMR spectroscopy, electrochemistry as well as single crystals X-ray diffraction in the case of 1 and 4. The compounds have also been studied for their hydrolysis in aqueous environment, and for the catalytic regioselective reduction of NAD+ to 1,4-NADH in aqueous solution with sodium formate as hydride source. Moreover, the stoichiometric and catalytic oxidation of 1,4-NADH have also been investigated by UV-Visible spectrophotometry and NMR spectroscopy. Overall, initial structure-activity relationships could be inferred which point towards the influence of the extension of the aromatic N^N ligand in the cationic complexes 1-3 on the TH in both reduction/oxidation processes. The neutral complex 4, featuring a picolinamidate N^N ligand, stands out as the most active catalyst for the reduction of NAD+, while being completely inactive towards NADH oxidation. The compound can also convert pyruvate into lactate in the presence of formate, albeit with scarce efficiency. In any case, for all compounds, Ru(II) hydride intermediates could be observed and even isolated in the case of complexes 1-3. Together, insight from the kinetic and electrochemical characterization suggests that, in the case of Ru(II) complexes 1-3, catalytic NADH oxidation sees the H-transfer from 1,4-NADH as the rate limiting step, whereas for NAD+ hydrogenation with formate as the H-donor, the rate limiting step is the transfer of the ruthenium hydride to the NAD+ substrate. The latter is further modulated by the presence of di-cationic aquo- or mono-cationic hydroxo-species of complexes 1-3. Instead, compound 4, stable with respect to hydrolysis in aqueous solution, appears to operate via a different mechanism. Finally, the anticancer activity and ability to form reactive oxygen species (ROS) of complexes 1-3 have been studied in cancerous and non-tumorigenic cells in vitro. Noteworthy, the conversion of aldehydes to alcohols could be achieved by the three Ru(II) catalysts in living cells, as assessed by fluorescence microscopy. Furthermore, the formation of Ru(II) hydride intermediate upon treatment of cancer cell extracts with complex 3 has been detected by 1H NMR spectroscopy. Overall, this study paves the way to the application of non-arene based organometallic complexes as TH catalysts in biological environment.
Abstract Electrocatalytic hydrogenation of 1‐octene as non‐activated model substrate with neutral water as H‐donor is reported, using [( t Bu PCP)Ir(H)(Cl)] ( 1 ) as the catalyst, to form octane with high faradaic efficiency (FE) of 96 % and a k obs of 87 s −1 . Cyclic voltammetry with 1 revealed that two subsequent reductions trigger the elimination of Cl − and afford the highly reactive anionic Ir(I) hydride complex [( t Bu PCP)Ir(H)] − ( 2 ), a previously merely proposed intermediate for which we now report first experimental data by mass spectrometry. In absence of alkene, the stoichiometric electrolysis of 1 in THF with water selectively affords the Ir(III) dihydride complex [( t Bu PCP)Ir(H) 2 ] ( 3 ) in 88 % FE from the reaction of 2 with H 2 O. Complex 3 then hydrogenates the alkene in classical fashion. The presented electro‐hydrogenation works with extremely high FE, because the iridium hydrides are water stable, which prevents H 2 formation. Even in strongly alkaline conditions (Bu 4 NOH added), the electro‐hydrogenation of 1‐octene with 1 also proceeds cleanly (89 % FE), suggesting a highly robust process that may rely on H 2 O activation, reminiscent to transfer hydrogenation pathways, instead of classical H + reduction. DFT calculations confirmed oxidative addition of H 2 O as a key step in this context.
Abstract Stable reference electrodes (REs) are crucial for reliable voltammetry, controlled potential electrosynthesis, or spectro‐electrochemistry. Yet, inferior pseudo‐REs, such as plain Ag wire are often used, because commercial REs are expensive, may degrade or contaminate under required conditions, or don't fit geometric restrictions of custom setups. Addressing such cases, we report construction, benchmarking and utilization of easy to make, cheap (<1 €), and robust miniature REs from pasteur pipettes, molecular sieve beads and Ag wire. Excellent potential precision and accuracy with at least 1 week device stability was obtained for aqueous Ag/AgCl REs with 3 M NaCl in H 2 O and anhydrous 0.1 M TBACl in MeCN inner‐electrolytes. Even in alkaline 1 M NaOH electrolyte, where initial Ag/AgCl REs quickly convert to Ag/Ag 2 O (1 M NaOH) REs, perfect potential precision, accuracy, and at least 1 week stability are demonstrated. According to experimental needs, miniature REs can be built with many organic and aqueous electrolytes. For these custom use‐cases, we report guidelines to calibrate absolute RE potential, and to assess device precision, accuracy and stability. Finally, electro‐hydrogenation of styrene on nickel electrodes exemplifies superiority of Ag/AgCl REs over Ag wire pseudo‐REs, affording more reliable electrolysis current, cell potential and potential dependent Faraday efficiency.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
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