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A convenient synthetic route to 4-arylpyridine-2,6-dicarboxylic acid has been proposed on the basis of substituted benzaldehydes according to the Krohnke method, followed by oxidation of methyl group.

Two new amphiphilic platinum(II) complexes, [Pt(2-(4-fluorophenyl)-5-(4-dodecyloxyphenyl)pyridine) (acac)] (Pt-1) and [Pt(2-(4-dodecyloxyphenyl)-5-(thien-2-yl)-c-cyclopentenepyridine) (acac)] (Pt-2), where acac is acetylacetonate, were synthesized and characterized. Apart from conventional phosphorescence of single molecules (ME—monomer emission), complexes Pt-1 and Pt-2 also exhibit excimer emission (EE) when embedded into phospholipid vesicles, that is assigned to emissive Pt–Pt excimers. The EE intensity in vesicular media appeared to depend on the viscosity of the vesicles and the concentration of the embedded complex. Differences in the EE properties of complexes Pt-1 and Pt-2 are correlated with the energies of the π-character frontier orbitals defined by the design of the cyclometalating phenylpyridine ligand. Higher energies of the frontier π-orbitals (HOMO and LUMO) naturally promote stronger π–π interactions, thus obstructing the PtII–PtII interaction.

By means of “1,2,4-triazine” strategy new 2,2′-bipyridine-type lanthanide ligands, i.e. 5-aryl-2,2′-bipyridine-6′-carboxylic acids and its cycloalkane-annulated derivatives, i.e. 6-(4-aryl-6,7-dihydro-5H-cyclopenta[c]pyridin-1-yl)picolinic acids, have been synthesized, and their luminescent neutral lanthanide complexes have been prepared. Due to the presence of an annulated cyclopentene fragment the last ligand lanthanide complexes exhibited high solubility in non-polar solvents. In comparison with early published chelates the herein reported complexes exhibit improved photophysical properties in non-polar solvents namely more pronounced bathochromic shift in absorption spectra and higher europium luminescence quantum yields. The structures of two europium complexes have been confirmed based on X-ray diffraction analysis.

Luminescent lanthanide complexes of 4-aryl-1-(2-pyridyl)-6,7-dihydro-5H-cyclopenta[c]pyridine-3-carboxylic acids and 5-tolyl-2,2′-bipyridine-6-carboxylic acid were prepared. The ligands were synthesized using the “1,2,4-triazine” methodology. In one case, the opportunity of copper complex preparation, followed by exchange of the metal cation from copper to europium to overcome synthetic difficulties, was demonstrated. The ability of tuning the properties of the complexes by means of varying the structure of the ligands was demonstrated. In particular, the introduction of the cyclopentene moiety allowed the preparation of highly soluble complexes in non-polar organic solvents.

Based on ‘1,2,4-triazine’ methodology, a new lanthanide( iii ) ligand, 5-(4-dodecyloxyphenyl)-2,2 ’ -bipyridine-6 ’ -carboxylic acid, was synthesized, and its luminescent europium( iii ) complex was prepared. The solubility in organic solvents for both the ligand and its europium complex was provided by the presence of a long aliphatic dodecyl moiety. The photophysical properties of the europium( iii ) complex were examined, the fluorescence quantum yield and the europium cation luminescence quantium yield were determined.

A highly soluble neutral cyclometallated Pt complex of C N N type was prepared and its self-aggregation in CDCl3 solution was studied by NMR dilution method. It was shown that 195Pt NMR can be used to quantify aggregation and therefore be potentially applied to many other Pt complexes. Two-dimensional HMBC 1H-195Pt technique increases sensitivity of experiment and allowed recording 195Pt chemical shift values at concentration as low as 1 mM and it was observed that opposite to shielding of aromatic protons, Pt nucleus is deshielded upon increase of concentration. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Photophysical properties of four new platinum(II) complexes comprising extended ppy (Hppy = 2-phenylpyridine) and thpy (Hthpy = 2-(2'-thienyl)pyridine) cyclometalated ligands and acetylacetonate (acac) are reported. Substitution of the benzene ring of Pt-ppy complexes 1 and 2 with a more electron-rich thiophene of Pt-thpy complexes 3 and 4 leads to narrowing of the HOMO-LUMO gap and thus to a red shift of the lowest energy absorption band and phosphorescence band, as expected for low-energy excited states of the intraligand/metal-to-ligand charge transfer character. However, in addition to these conventional spectral shifts, another, at first unexpected, substitution effect occurs. Pt-thpy complexes 3 and 4 are dual emissive showing fluorescence about 6000 cm(-1) (∼0.75 eV) higher in energy relative to the phosphorescence band, while for Pt-ppy complexes 1 and 2 only phosphorescence is observed. For dual-emissive complexes 3 and 4, ISC rates kISC are estimated to be in order of 10(9)-10(10) s(-1), while kISC of Pt-ppy complexes 1 and 2 is much faster amounting to 10(12) s(-1) or more. The relative intensities of the fluorescence and phosphorescence signals of Pt-thpy complexes 3 and 4 depend on the excitation wavelength, showing that hyper-intersystem crossing (HISC) in these complexes is observably significant.

Rational design of selective and sensitive 'off-on' fluorescent probes for Zn(II) cations exploiting both PET and ICT mechanisms of sensing is illustrated by the synthesis and application of 5-aryl-2,20-bipyridines modified with a dipicolylaminomethyl fragment. The aryl substituent provides tuning of the properties. © 2012 Elsevier Ltd. All rights reserved.

The 'triazine' methodology for the synthesis of functionalized pyridine ligands proved to be a convenient method for the preparation of a luminescent Pt(II) complex. The key ligand can be assembled easily starting from readily accessible reagents. Further cycloplatination and post-functionalization led to the ready-to-go luminescent 'tag' 2 for peptide labeling. © 2012 Elsevier Ltd. All rights reserved.

A wide series of substituted bipyridines were obtained through the synthesis of 1,2,4-triazines and their aza Diels–Alder reactions. The reported method facilitates the synthesis of functionally diverse bipyridines that provides fine-tuning of photophysical properties of new ligands and their Zn(II) and Ru(II) complexes. Some of substituted bipyridines exhibit ‘off–on’ fluorescence response toward Zn2+ cations.

The synthesis of pyridine and bipyridine derivatives of m-carborane via aza Diels–Alder reaction of 1,2,4-triazines with 9-allyl m-carborane and their structural characterization and photophysical properties are described. The products 3- and 4-(m-carborane-9-ylmethyl)-2,2′-bipyridines form Zn(II) complexes on reaction with ZnCl2, which increases significantly their fluorescence intensity.

Meanwhile 5-aryl-6-cyano-2,2′-bipyridines are very stable towards various nucleophiles, addition copper(II) chloride to the reactional mixture facilitates nucleophilic addition to the cyano group dramatically. The cyanobipyridines react easily with water, methanol, ethanolamine in the presence of CuCl2 yielding well-crystallized complexes containing carboxylates, carboximidates or carboxamidines as ligands. 5-Cyano-1,2,4-triazines are more active in the reactions due to higher electron-withdrawing properties of this heterocycle. Due to the same reason acetylene moiety of 5-ethynyl-3-pyridyl-1,2,4-triazine adds water quite easily but in the presence of copper chloride as well.

The new N,N,O-ligand 6-phenyl-3-(2-pyridyl)-5-(trifluoroacetylmethylidene)-4,5-dihydro-1,2,4-triazine (L) was synthesized, and its complex with CuII was prepared. The structure of the complex was established by X-ray diffraction. Magnetic measurements and ESR studies were carried out. Ab initio calculations of the electronic structure, exchange interactions, and the effective magnetic moment were performed. The intrastack polarization effect between the π states of 1,2,4-triazine and the d states of copper was revealed and numerically validated.

A procedure has been proposed for the synthesis of pyridines from 1,2,4-triazine derivatives and bicyclo[2.2.1]hepta-2,5-diene under high pressure in the presence of lithium perchlorate as catalyst.

Readily available 5-aryl-2,2′-bipyridines and their derivatives are a unique family of fine-tunable chromophores, where changes in the substitution patterns, solvation and coordination are translated into dramatic spectral changes. They exhibit sensitive and selective response to zinc ions with dramatic increases in emission intensity or significant red-shifts of emission maxima.

A consecutive aromatic nucleophilic substitutions of hydrogen in 1,2,4-triazine 4-oxides and an aza Diels−Alder reaction is a versatile route to carborane-functionalized bi- and terpyridines and their 1,2,4-triazine analogues. The heterocycles facilitate deboronation of the substituted carboranes, and the carborane moiety has a significant influence on the coordination properties of the ligands as well.

An efficient strategy for the synthesis of functionalized 2,2′-bipyridines is reported. The strategy is based on readily available 3-pyridyl-1,2,4-triazine 4-oxides and uses a reaction sequence of nucleophilic substitution of hydrogen and aza Diels–Alder reaction.

A series of substituted 2-amino-3-ethoxycarbonylpyrazines containing indole, resorcinol, thiophenol, ethyl cyanoacetate, indandione, and antipyrine moieties was obtained via reactions of nucleophilic substitution of hydrogen in the initial 2-aminopyrazine-1-oxides. Some of the synthesized compounds inhibit the reproduction of measles viruses and exhibit a weak antiviral activity with respect to Marburg virus. However, most of the new substituted pyrazines are not cytotoxic and exhibit no activity against ortho-poxviruses and measles viruses.

5-Acylmethyl-3-(2-pyridyl)-1,2,4-triazines are easily prepared from accessible 3-(2-pyridyl)-1,2,4-triazine 4-oxides by reaction of the latter with acetophenone or trifluoroacetone in the presence of NaH. The compounds obtained behaved as efficient ligands in reaction with CuCl2 furnishing dimeric neutral complexes with Cu(II) whose structure was investigated by means of X-ray diffraction analysis.

An efficient strategy for the synthesis of new pyridine and 1,2,4-triazine derivatives starting from available 6-aryl-3-chloromethyl-1,2,4-triazine 4-oxides was proposed. The deoxygenative nucleophilic hydrogen substitution in the triazine-oxide ring, nucleophilic substitution of the chlorine atom in the side chain, and transformations of the 1,2,4-triazine ring into the pyridine ring via the inverse-electron-demand Diels-Alder reactions, being used in different orders, are a rather flexible tool for the functionalization of the titled heterocycles. The cyanide anion, indoles, thiophenols, amines, and triphenylphosphine were used as nucleophiles. The direct introduction of indole residues into the 1,2,4-triazine ring followed by the substitution of the chlorine atom by a residue of the primary or secondary aliphatic amine was found to be the most convenient method for the library synthesis.

The reactions of 1-lithia-1,2- or 1,7-dicarba-closo-dodecaborane with 1,2,4-triazine 4-oxides can follow two competitive pathways: deoxygenative nucleophilic substitution of hydrogen to form 1-(1,2,4-triazin-5-yl)-1,2- or 1,7-dicarba-closo-dodecaboranes and the transformation of the 1,2,4-triazine ring into the triazoline ring giving rise to 1-(2-acetyl-1-aroyl-3-aryl-1,2,4-triazolin-5-yl)-1,2-dicarba-closo-dodecaboranes. Introduction of the electron-withdrawing triazine ring into the carborane cage substantially facilitates deboronation to give 1-(1,2,4-triazin-5-yl)-1,2-1,7-dicarba-nido-undecaboranes.

The synthesis of substituted terpyridines via triazine intermediates is described. The intermediates allow the facile introduction of cyano groups, which are converted into amino diacetic acid moieties. Together with the terpyridine nitrogen atoms they shield coordinated europium(III) ions completely from the solvent. The terpyridines may be further functionalized for specific applications using ligands, bromo substituents, and standard transition metal mediated coupling methods. This makes the title compounds valuable intermediates for the preparation of luminescent probes in biochemistry, medicinal diagnostics or materials science.

The 3,6-diaryl-1,2,4-triazine 4-oxides 1a−e undergo a nucleophilic substitution of hydrogen with the α-halomethyl aryl sulfones 2, 3 and 7 by two alternative pathways: vicarious nucleophilic substitution (VNS) and/or an intramolecular deoxygenative process. The former pathway is found to dominate in the reaction of 1 with bromomethyl tolyl sulfone (7) yielding the 5-tosylmethyl-1,2,4-triazine 4-oxides 6, while the reaction with the chloromethyl aryl sulfones 2 and 3 leads to the 5-arylsulfonylchloromethyl-1,2,4-triazines 4 and 5, respectively, as the products of deoxygenative substitution. The reaction of 6-phenyl-1,2,4-triazine 4-oxide (1f) with the chloromethyl aryl sulfones 2 and 3 proceeds differently. At low temperature (−75 C) the products of the VNS reaction at position 5 (6f and 12f) are formed. At room temperature the 7-chloro-1-hydroxy-3-phenyl-7-(arylsulfonyl)-1,4,5-triazahepta-1,3,6-trienes 10f and 11f are obtained by addition of the carbanions 2 and 3 in the 3-position, followed by the ring opening of the resulting σ adduct. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

The scope of functionalization of 1,2,4-triazines can be considerably extended via successive nucleophilic substitution of hydrogen (SN H) and ipso-substitution. A convenient procedure has been developed for direct cyanation of 1,2,4-triazine 4-oxides with acetone cyanohydrin in the presence of triethylamine. The cyano group in the resulting 5-cyano-1,2,4-triazines is readily replaced by reactions with various aliphatic alcohols and amines.

A series of three Pt(II) complexes with a doubly cyclometalating terdentate ligand L1, L1H2 = 3,6-bis(p-anizolyl)-2-carboranyl-pyridine, and diethyl sulfide (1), triphenylphosphine (2), and t-butylisonitrile (3) as ancillary ligands were synthesized. X-ray diffraction studies of 1 and 2 show a coordination of the L1 ligand in a C-N-C mode in which the bulky and rigid o-carborane fragment is cyclometalated via a C atom. Importantly, no close intermolecular Pt-Pt contacts occur with this ligand type. The new Pt(II) pincer complexes display very high luminescence quantum yields at decay times of several tens of μs even in solution under ambient conditions. On the basis of the low-temperature (T = 1.3 K) emission decay behavior, the emission is assigned to a ligand centered triplet excited state (3)LC with small (1,3)MLCT admixtures. Because the phosphorescence is effectively quenched by molecular oxygen, optical sensors operating in a wide range of oxygen pressure can be developed. Owing to the very high luminescence quantum yields, the new materials might also become attractive as emitter materials for diverse optoelectronic applications.

This review covers the work published in the calendar year 2013. Novel reaction chemistry and new ring synthetic methods for 1,2,3-triazines, 1,2,4-triazines, 1,3,5-triazines, and tetrazines are reviewed.

1,2,4-Triazine 4-oxides were found to enter into the reactions of nucleophilic substitution of hydrogen with S-nucleophiles (arenethiols) in the presence of acylating agents and trifluoroacetic acid. The reactions proceeded with loss of the N-oxide function to form 5-arylthio-1,2,4-triazines. 2-Amino-3-ethoxycarbonylpyrazine 1-oxides and 2-amino-4-oxopterin 8-oxides react with arenethiol analogously.

Addition of hydroxylamine at the 3 position of 6-aryl-5-amino-1,2,4-triazine-4-oxides initiates the amidine rearrangement resulting in 6-aryl-5-hydroxylamine-1,2,4-triazines, as confirmed by an experiment with 15 N-labeling.

Reaction of 6-aryl-3-dimethylamino-1,2,4-triazine 4-oxides with KCN results in constriction of the heterocycle and formation of 3-amino-5-aryl-4-nitrosopyrazoles. Several methods including labelled K13C15N are used to demonstrate that the reaction proceeds through the ANRORC mechanism.

A new, efficient methodology for the synthesis of substituted thienylpyridines includes the synthesis of 3-thienyl-1,2,4-triazines using simple heterocyclisation followed by easy transformation of the triazine ring to a pyridine through an aza Diels–Alder approach. A variety of substituted pyridines can be easily achieved using cheap, commercially available reagents such as bromoacetylarenes, aroyl hydrazides, norbornadiene, and enamines in various combinations. New thienylpyridines form phosphorescent cyclometallated Pt complexes.

This review deals with the synthesis and properties of 1,2,4-triazine N-oxides and their annelated derivatives. Methods for the incorporation of the N-oxide group in the 1,2,4-triazine ring by direct oxidation and cyclisation involving nitro, nitroso, and isonitroso groups are discussed. Reactions of 1,2,4-triazine N-oxides with electrophiles, nucleophiles, and dienophiles are considered. The effect of the N-oxide group on the chemical shifts in the 1H, 13C, and 14N NMR spectra and the specific features of decomposition of 1,2,4-triazine N-oxides under electron impact are analysed. The bibliography includes 107 references.

A new route to substituted bipyridines based on a new method for the synthesis of substituted 3-pyridyl-1,2,4-triazines and their aza-Diels–Alder reactions is shown to be an efficient strategy for the preparation of structurally diverse bipyridine ligands.

The crystallographic data, NMR and UV-VIS spectra and luminescence of the heteroleptic complexes [Ru(L)(bpy)2](PF6)2 (L = 5-aryl-6-cyano-2,2’-bipyridine) are described.

Two types of ring transformations of 1,2,4-triazine to pyridazine were found: 1,2,4-triazine 4-oxides bearing ortho-halophenyl in the 3-position react with phenylacetonitrile under basic conditions to form 3-(2-oxyphenyl)[1,2,4]triazolo[4,3-b]pyridazines, while the reactions of 3,6-diaryl-1,2,4-triazin 4-oxides with sodium ethyl cyanoacetate afforded 2-aminopyridazines.

Pyrimidine 1-oxides with cyanamide afforded 2-ureidopyrimidines as the result of the nucleophilic substitution of hydrogen, whereas the formation of similar 2-trichloroacetylaminopyrimidines occurs as dipolar 1,3-cycloaddition of the same oxides to trichloroacetonitrile under much more drastic conditions and in lower yields.

The review covers work published in the calendar year 2010. Novel reaction chemistry and new ring synthetic methods for 1,2,3-triazines, 1,2,4-triazines, 1,3,5-triazines, tetrazines, and purines are reviewed.

A new methodology for the synthesis of functionalised 2,2′:6′,2′′-terpyridine systems is suggested: sequential synthesis of the heterocyclic assemblies based on the 1,2,4-triazine ring, direct introduction of the cyano group in the 1,2,4-triazine ring and the transformation of the latter to a pyridine ring via an aza-Diels–Alder reaction.

Two newly prepared oligothienylpyridines, 5-(2-pyridyl)-5'-dodecyl-2,2'-bithiophene, HL(2), and 5-(2-pyridyl)-5''-dodecyl-2,2':5',2''-ter-thiophene, HL(3), bind to platinum(II) and iridium(III) as N∧C-coordinating ligands, cyclometallating at position C(4) in the thiophene ring adjacent to the pyridine, leaving a chain of either one or two pendent thiophenes. The synthesis of complexes of the form [PtL(n)(acac)] and [Ir(L(n))(2)(acac)] (n = 2 or 3) is described. The absorption and luminescence properties of these four new complexes are compared with the behavior of the known complexes [PtL(1)(acac)] and [Ir(L(1))(2)(acac)] {HL(1) = 2-(2-thienyl)pyridine}, and the profound differences in behavior are interpreted with the aid of time-dependent density functional theory (TD-DFT) calculations. Whereas [PtL(1)(acac)] displays solely intense phosphorescence from a triplet state of mixed ππ*/MLCT character, the phosphorescence of [PtL(2)(acac)] and [PtL(3)(acac)] is weak, strongly red shifted, and accompanied by higher-energy fluorescence. TD-DFT reveals that this difference is probably due to the metal character in the lowest-energy excited states being strongly attenuated upon introduction of the additional thienyl rings, such that the spin-orbit coupling effect of the metal in promoting intersystem crossing is reduced. A similar pattern of behavior is observed for the iridium complexes, except that the changeover to dual emission is delayed to the terthiophene complex [Ir(L(3))(2)(acac)], reflecting the higher degree of metal character in the frontier orbitals of the iridium complexes than their platinum counterparts.

International audience; The ‘triazine’ methodology for the synthesis of functionalized bipyridine ligands proved to be a convenient method for the preparation of luminescent Eu(III) complexes. The approach allows flexible construction of chromophore and coordination sphere with control of photophysical properties. Europium(III) complexes [Eu1]–[Eu5] prepared in this way exhibit intense long-life metal-centered luminescence in aqueous media. The aromatic substituent in the position 5 of bipyridine has a significant influence on luminescence parameters and is used to introduce functionality for bioconjugation. The complexes [Eu4] and [Eu5] bearing primary amine groups are ready-to-go luminescent ‘tags’ for peptide labeling.

A new method for the synthesis of 1,2,4-triazine 4-oxide derivatives has been suggested; this method includes the reaction of isonitrosoketone hydrazones 2 with aldehydes followed by oxidation of 3-R-3,4-dihydro-4-hydroxy-6-phenyl-1,2,4-triazines 3 by lead tetraacetate.