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Preparation of a series of terpyridyl ligands bearing different substituents recently led to the synthesis of new cobalt-bisterpyridyl complexes spanning over a wide range of redox potentials. In this work, we describe the catalytic properties of these complexes for the electroreduction of protons into hydrogen (hydrogen evolution reaction (HER)) in acetonitrile. The substituents of the ligands were found to greatly affect the catalytic performances of the systems, in terms of stability and overpotential. Interestingly, systems based on dimethylamino-terpyridine derivatives perform HER with high efficiency, low overpotential and excellent stability. Density functional theory calculations were used to provide insights into the reaction mechanism of HER catalyzed by these systems, highlighting the role of the ligand for proton activation.

G-quadruplexes formed by nucleic acids are implicated in pathologies ranging from cancers to neurodegenerative diseases. We evaluated interactions of 29 bi- and terpyridine derivatives with G-quadruplexes and duplexes. FRET-melting, circular dichroism, and (1) H NMR spectroscopy showed that one terpyridine derivative interacted very selectively with G-quadruplexes. This G-quadruplex ligand inhibited helicase activity and should influence G-quadruplex-related biological processes.

The scope of a flexible route to unsymmetrically functionalized bipyridines is described. Starting from 1,3-diketones 1a–e, the corresponding β-ketoenamines 2a–e were converted into different β-ketoenamides 3a–g by N-acylation with 2-pyridinecarboxylic acid derivatives. These β-ketoenamides were treated with a mixture of TMSOTf and Hünig’s base to promote the cyclocondensation to 4-hydroxypyridine derivatives. Their immediate O-nonaflation employing nonafluorobutanesulfonyl fluoride provided the expected 4-nonafloxy-substituted bipyridine derivatives 5a–g in moderate to good overall yields. The bipyridyl nonaflates are excellent precursors for palladium-catalyzed reactions as demonstrated by representative Suzuki and Sonogashira couplings. Thus, a library of specifically substituted bipyridine derivatives was generated, showing the versatility of the simple 1,3-diketone-based approach to this important class of ligands.

On the basis of an easy access to 2,2′:6′,2″-terpyridine derivatives bearing 6- and 6″-methyl groups, the functionalization of these substituents was investigated. The direct oxidation or bromination of the methyl groups was not feasible on larger scale; however, a two-step process employing oxidation to the pyridine N-oxides followed by acetic anhydride promoted rearrangement (Boekelheide reaction) furnished the corresponding terpyridine derivatives with 6- and 6″-acetoxymethyl groups in reasonable overall yields. These are excellent precursors for the synthesis of tetra- and pentadentate ligands, as demonstrated by several examples. This approach allowed access to a library of new functionalized 2,2′:6′,2″-terpyridine derivatives.

We describe a novel approach to highly functionalized ferrocenyl-substituted pyridine derivatives. Key reaction is the cyclocondensation of an easily available β-ketoenamide bearing an N-ferrocenylcarbonyl group. The resulting 4-hydroxypyridine derivatives were converted into the corresponding pyrid-4-yl nonaflates, which were employed in a range of palladium-catalyzed coupling reactions. The reaction sequence delivers a library of new 2-ferrocenyl-substituted pyridine derivatives, including compounds with two ferrocene moieties, a butadiyne derivative, a 2,4-bisferrocenylpyridine, and a compound with a thiophene core and two 2-ferrocenylpyridine groups. The compounds were investigated by UV/Vis spectroscopy and by (spectro)electrochemistry. The (spectro)electrochemical results conclusively prove that the reaction takes place through a ferrocene-centered oxidation and a reduction based on the heterocyclic part of the molecule. Intriguing effects on the electrochemical responses of these systems upon changing the electrolyte are also observed.

Starting from easily accessible precursors we describe the preparation of a series of branched oligo(2-thienyl)- and oligo(2,2′-bithienyl)-substituted pyridine derivatives. With palladium-catalyzed cross-coupling reactions of pyridyl nonaflates/triflates as key steps we synthesized 2,6-di(2-thienyl)pyridines bridged by thiophene or benzene rings. By selective bromination of 2,6-di(2-thienyl)pyridine and 2,4,6-tri(2-thienyl)pyridine and subsequent coupling reactions an access to oligo(2,2′-bithien-5-yl)-substituted pyridine derivatives was gained. The constitution and solid state conformation of 2,6-bis(2,2’-bithien-5-yl)pyridine was determined by X-ray analysis. This series of new pyridine-thiophene conjugates was systematically investigated by UV/vis spectroscopy. Large Stokes shifts in the neutral and protonated form were observed. The electrochemical oxidation of two typical compounds was studied, however, these oxidations were irreversible forming a polymeric film at the anode. As a selected example, a thiophene-bridged 2,6-di(2-thienyl)pyridine derivative was also investigated by scanning tunneling microscopy showing an interesting self-assembly into a highly ordered monolayer on highly oriented pyrolytic graphite.

The intramolecular condensation of β-ketoenamides to 2- and/or 4-pyridone derivatives using either KOt-Bu or trimethylsilyl trifluoromethanesulfonate/Hunig’s base was investigated. Subsequent nonaflation of the cyclization products allowed a facile purification and further functionalization through Suzuki–Miyaura couplings leading to new highly substituted pyridine derivatives. The dependence of the regioselectivity of cyclocondensation on the structure of the β-ketoenamides is discussed.

The combination of 2,2ʹ:6ʹ,2″-terpyridines (tpy) and RuII is known to deliver molecular and supramolecular assemblies with remarkable properties. We present here new RuII complexes with modified tpy ligands substituted with varying numbers of dimethlyamino groups. Electrochemistry shows that the incorporation of the strongly electron-donating groups on the tpy ligands leads to a negative shift of the RuII oxidation potential by close to 1 V. The reductive electrochemical responses are strongly dependent on the nature of the working electrode, with glassy carbon and gold working electrodes showing the best results. These observations led us to develop a modified Optically Transparent Thin Layer Electrochemical (OTTLE) cell, based on a gold working electrode. The use of UV-vis-NIR spectroelectrochemical results with that OTTLE cell, together with simulations of the cyclic voltammograms, allow us to characterize four reduction steps in these complexes, the final two of which lead to bond activations at the ruthenium center. This observation is to the best of our knowledge unprecedented in coordinatively saturated complexes of type [Ru(tpy)2]2+. The various redox states of the complexes were characterized by EPR spectroelectrochemistry and through DFT calculations. The results presented here establish these substituted tpy ligands as highly attractive ligands in coordination chemistry, and display the utility of a gold-based OTTLE cell for spectroelectrochemical measurements.

International audience; Terpyridine ligands are widely used in chemistry and material sciences owing to their ability to form stable molecular complexes with a large variety of metal ions. In that context, variations of the substituents on the terpyridine ligand allow modulation of the material properties. Applying the Stille cross-coupling reaction, we prepared with good yields a new series of terpyridine ligands possessing quinoline-type moieties in ortho, meta, and para positions and dimethylamino substituents at central or distal positions. The corresponding cobalt(II) complexes were synthesized and fully characterized by elemental analysis, single-crystal X-ray crystallography, mass spectrometry, and UV–vis, 1H NMR, and Fourier transform infrared (FT-IR) spectroscopy as well as by cyclic voltammetry (CV). Density functional theory (DFT) calculations were performed to investigate the electronic structure of all the Co(II) bis-terpyridyl molecular complexes. In this work, we show that terpyridine ligand functionalization allows tuning the redox potentials of the Co(III)/Co(II), Co(II)/Co(I), and Co(I)/Co(I) (tpy)•– couples over a 1 V range.

The scope of a flexible route to unsymmetrically functionalized bipyridines is described. Starting from 1,3-diketones 1a–e, the corresponding β-ketoenamines 2a–e were converted into different β-ketoenamides 3a–g by N-acylation with 2-pyridinecarboxylic acid derivatives. These β-ketoenamides were treated with a mixture of TMSOTf and Hunig’s base to promote the cyclocondensation to 4-hydroxypyridine derivatives. Their immediate O-nonaflation employing nonafluorobutanesulfonyl fluoride provided the expected 4-nonafloxy-substituted bipyridine derivatives 5a–g in moderate to good overall yields. The bipyridyl nonaflates are excellent precursors for palladium-catalyzed reactions as demonstrated by representative Suzuki and Sonogashira couplings. Thus, a library of specifically substituted bipyridine derivatives was generated, showing the versatility of the simple 1,3-diketone-based approach to this important class of ligands.

Palladium-catalyzed couplings of pyrid-4-yl nonaflates with methyl diazoacetate are described. After optimization of the reaction conditions the scope of the transformation proved to be fairly broad and a series of pyrid-4-yl-substituted methyl diazoacetates was prepared in generally high yields.

A convenient method for the synthesis of dimethylamino functionalized terpyridines by assembling of the terminal pyridine units using 2,6-pyridinedicarboxylic acids as starting materials is presented.

The intramolecular condensation of β-ketoenamides to 2- and/or 4-pyridone derivatives using either KOt-Bu or trimethylsilyl trifluoromethanesulfonate/Hunig’s base was investigated. Subsequent nonaflation of the cyclization products allowed a facile purification and further functionalization through Suzuki–Miyaura couplings leading to new highly substituted pyridine derivatives. The dependence of the regioselectivity of cyclocondensation on the structure of the β-ketoenamides is discussed.

A versatile method for the synthesis of functionalized 2,2':6',2''-terpyridines by assembly of the terminal pyridine rings is presented. The cyclization precursors-bis-β-ketoenamides-are prepared from 4-substituted 2,6-pyridinedicarboxylic acids and acetylacetone or its corresponding enamino ketone. Treatment with trimethylsilyl trifluoromethanesulfonate induces a twofold intramolecular condensation providing an efficient access to 4,4''-di- and 4,4',4''-trifunctionalized 6,6''-dimethyl-2,2':6',2''-terpyridines. Using this method, hitherto unknown 4,4''-bis(dimethylamino)- and 4,4',4''-tris(dimethylamino)terpyridines have been prepared that show remarkably high calculated Lewis basicities.

Starting from an easily available pyridinol derivative, a route to penta(2-thienyl)pyridine and related symmetrical compounds is reported. Key reactions are activation of the pyridine core and metal-catalyzed couplings proving the efficacy of these methods even in sterically highly encumbered systems. UV/vis and fluorescence spectra as well as first cyclovoltametric measurements of the synthesized novel thiophene–pyridine conjugates are reported.

Optimized conditions are developed to give the corresponding pyridyl-substituted diazoacetates.

E/Z-Mixtures of N-acylated-β-ketoamides are prepared from the corresponding nitriles and used as efficient precursors for the TmsOTf-promoted cyclization to 4-hydroxypyridine derivatives.

Herein, we describe our attempts to systematically prepare a series of oligo(2-thienyl)-substituted pyridine derivatives. The crucial starting material, a β-alkoxy-β-ketoenamide, is easily available on a large scale by the reaction of lithiated methoxyallene with thiophene-2-carbonitrile and thiophene-2-carboxylic acid. This three-component reaction is followed by intramolecular cyclization to yield the suitably functionalized 2,6-di(2-thienyl)-substituted pyridine derivates. The two oxygen atoms allow the programmed activation of positions C-3, C-4, or C-5 of the pyridine ring to perform palladium-catalyzed coupling reactions with thiophene-2-boronic acid or 2-(tributylstannyl)thiophene, and alternatively, reductive removal of groups. With this concept, we were able to prepare five pyridine derivatives with 2-thienyl substituents in the 2,6-, 2,3,6-, 2,4,6-, 2,3,4,6-, and 2,3,5,6-positions. 2,3,4,5,6-Penta(2-thienyl)pyridine was not available with our methods. The UV/Vis and fluorescence spectra of all pyridines were recorded and showed a dependence on the substitution pattern and protonation state. For the protonated 2,3,5,6-tetra(2-thienyl)-substituted pyridine, a Stokes shift of about 180 nm with an emission at 515 nm was observed.

A three-component reaction with lithiated alkoxyallenes, nitriles, and perfluorinated carboxylic acids as precursors led to a series of perfluoroalkyl- or perfluoroaryl-substituted 4-hydroxypyridine derivatives. These compounds were converted into 4-pyridyl nonaflates which can be employed as versatile building blocks for the synthesis of π-conjugated compounds with use of palladium-catalyzed couplings. Suzuki reactions at C-4 and C-3 of the pyridine ring proceeded with moderate to high yields. In addition, Suzuki−Miyaura, Stille, or Buchwald−Hartwig coupling reactions have also been studied and afforded the corresponding highly substituted pyridine derivatives. Starting from an arylated propargylic ether the three-component reaction led to a pentasubstituted 4-hydroxypyridine derivative that could also be employed in palladium-catalyzed processes at C-4 and at C-3 of the pyridine core. With this simple approach the sterically highly crowded 3,4,5-triphenyl-substituted pyridine derivative 37a could be pre...