Pyrazine-based Ligand Research Articles

A Zr-hydroxamate metal-organic framework with intrinsic chelating sites for postsynthetic Pd metalation and Suzuki-Miyaura catalysis.

A highly crystalline and robust Zr-hydroxamate metal-organic framework (MOF) was prepared from a pyrazine-based ligand, featuring abundant N,N' chelating sites. High-degree Pd(II) metalation of the MOF was achieved through straightforward postsynthetic modification, with detailed coordination chemistry elucidated spectroscopically. The Pd-functionalized MOF was then studied as a heterogeneous Suzuki-Miyaura catalyst, through combined experimental/computational methods.

Modulation of the structural information in shape-defined heterocyclic strands: the case of a (pyridine-hydrazone)2pyrazine ligand.

Metal ions (Ag+, Cd2+, Eu3+, Sm3+) and protons can, through coordination and protonation, modulate in three specific ways the structural information contained in the pyrazine-based heterocyclic strand L (obtained from 2,5-bis(methylhydrazino)pyrazine and 2 equivalents of 2-pyridinecarboxaldehyde), thus generating two linear rod-like conformations and a bent one. This conformational diversity is associated with a structural one that consists of two diprotonated forms (H2L(PF6)2 and H2L(CF3SO3)2), a polymeric architecture [AgL]n(CF3SO3)n, two rack-like complexes ([Eu2H2L3(CF3SO3)6](PF6)2 and [Sm2H2L3(CF3SO3)6](PF6)2) and a grid-like structure ([Cd4L4](CF3SO3)8).

Molecular Interactions of Pyrazine-Based Compounds to Proteins.

Pyrazine-based compounds are of great importance in medicinal chemistry. Due to their heteroaromatic nature, they uniquely combine properties of heteroatoms (polar interactions) with the properties of aromatic moieties (nonpolar interactions). This review summarizes results of a systematic analysis of RCSB PDB database focused on important binding interactions of pyrazine-based ligands cocrystallized in protein targets. The most frequent interaction of pyrazine was hydrogen bond to pyrazine nitrogen atom as an acceptor, followed by weak hydrogen bond with pyrazine hydrogen as donor. We also identified intramolecular hydrogen bonds within pyrazine ligands, π-interactions, coordination to metal ions, and few halogen bonds in chloropyrazines. In many cases the binding mode of the pyrazine fragment was complex, involving a combination of several interactions. We conclude that pyrazine as a molecular fragment should not be perceived as a simple aromatic isostere but rather as a readily interacting moiety of drug-like molecules with high potential for interactions to proteins.

Synthesis, Spectroscopic and Computational Studies of Some Metals Chelates with Chromene-2-one and Pyrazine-Based Ligands

The present paper deals with the synthesis of cobalt(II), nickel(II), copper(II) and cadmium(II) complexes with two bidentate ligands, L1 (3-(quinoxaline-2-yl)-coumarin) and L2 (2-methylene-2H-chromene-3-(methyl carbonimidic)thioanhydride). The L1 ligand was prepared by treating w-bromo-3-acetylcoumarin with 1,2-phenylenediamine whereas the ligand L2 was prepared through substitution reaction ofw-bromo-3-acetylcoumarin with potassium thiocyanate in ethanol medium. The confirmation of the structures for L1 and L2 were done by (C.H.N.S.) elemental analysis, FT-IR, NMR and mass spectra. The metal complexes of cobalt(II), nickel(II), copper(II) and cadmium(II), with L1 and L2, were prepared and isolated in the solid state then characterized by (C.H.N.M) elemental analysis, proton and carbon-13 NMR, FT-IR and mass spectra. Furthermore, the thermal analysis (TG-DSC) for some complexes assisted us in the elucidation of the suggested structures of complexes and confirmed their thermal stability. The results obtained from elemental analysis, magnetic susceptibility and thermal analysis confirmed that all metal complexes were formed in 2:1 molar ratio of ligand to metal with octahedral structures except cadmium(II) complexes which were in a tetrahedron geometry with 1:1 mole ratio. The complexes are found to be soluble in DMF and DMSO. The results obtained from TG-DSC analysis revealed that the metal complexes were thermally stable with point decomposition over 350 °C. The DFT/TDDFT calculations were carried out to provide the electronic structures and spectra of the compounds.

Construction of three 2-D silver(I) coordination architectures with a flexible pyrazine-based ligand

To investigate the impact of weak intermolecular interactions in construction of metal–organic frameworks, three silver(I) coordination complexes with the flexible N-heterocyclic ligand 1-((2-pyrazinyl)-1H-benzoimidazol-1-yl)methyl)-1H-benzotriazole (PBMBT), {Ag(C18H13N7)NO3} n (1), {Ag(C18H13N7)ClO4} n (2), and {Ag(C18H13N7)SO3CF3} n (3), were prepared under solvothermal conditions and structurally characterized by single-crystal X-ray diffraction, IR spectrum, and elemental analyses. Complexes 1–3 exhibit 2-D reticulate structures, and these 2-D layers are further connected into 3-D supramolecular motifs by π···π interactions and hydrogen bonds. Luminescence indicates that 1–3 show analogous fluorescent emissions compared with the PBMBT in the solid state at room temperature.

Solvent control: dinuclear versus tetranuclear complexes of a bis-tetradentate pyrimidine-based ligand

A new bis-tetradentate acyclic amine ligand L(Et) has been synthesized from 4,6-bis(aminomethyl)-2-phenylpyrimidine and 2-vinylpyridine. Dinuclear complexes, Mn(II)(2)L(Et)(MeCN)(H(2)O)(3)(ClO(4))(4) (1), Fe(II)(2)L(Et)(H(2)O)(4)(BF(4))(4) (2), Co(II)(2)L(Et)(H(2)O)(3)(MeCN)(2)(BF(4))(4) (3), Ni(II)(2)L(Et)(H(2)O)(4)(BF(4))(4) (4), Ni(II)(2)L(Et)(H(2)O)(4)(ClO(4))(4)·8H(2)O (4'), Cu(II)(2)L(Et)(BF(4))(4)·MeCN (5), Zn(II)(2)L(Et)(BF(4))(2)(BF(4))(2)·½MeCN (6), were obtained from 1 : 2 reactions of L(Et) and the appropriate metal salts in MeCN, whereas in MeOH tetranuclear complexes, Mn(II)(4)(L(Et))(2)(OH)(4)(ClO(4))(4) (7), Fe(II)(4)(L(Et))(2)(F)(4)(BF(4))(4)·5/2H(2)O (8), Co(II)(4)(L(Et))(2)(F)(4)(BF(4))(4)·3H(2)O (9), Ni(II)(4)(L(Et))(2)(F)(4)(BF(4))(4)·4H(2)O (10), Cu(II)(4)(L(Et))(2)(F)(4)(BF(4))(4)·3H(2)O (11) and Zn(II)(4)(L(Et))(2)(F)(4)(BF(4))(4) (12), result. Six complexes have been structurally characterized: in all cases each L(Et) is bis-tetradentate and provides a pyrimidine bridge between two metal centres. As originally anticipated, complexes 1, 4' and 6 are dinuclear, while 9, 10 and 12 are revealed to be tetranuclear, with two M(2)(L(Et))(4+) moieties bridged by two pairs of fluoride anions. Weak to moderate antiferromagnetic coupling between the metal centres is a feature of complexes 2, 3, 4, 8, 9 and 10. The dinuclear complexes 1-6 undergo multiple, mostly irreversible, redox processes. However, the pyrimidine-based dicopper(II) complex 5 undergoes a two electron quasi-reversible reduction, Cu(II)(2)→ Cu(I)(2), and this occurs at a more positive potential [E(m) = +0.11 V (E(pc) = -0.03 and E(pa) = +0.26 V) vs. 0.01 M AgNO(3)/Ag] than for either of the dicopper(II) complexes of the analogous pyrazine-based ligands.

Water-soluble tetrapodal N,O ligands incorporating soft N-heterocycles for the selective complexation of Am(iii) over Ln(iii)

A series of four water-soluble N,O-tetrapodal ligands derived from ethylenediamine, bearing hard acetate groups and soft N-heterocycles, either pyridine or pyrazine, was developed to study the impact of the softness of N-donors on the complexation properties with trivalent f ions. Two novel ligands of enhanced soft character, bearing three pyridines (L3py) or three pyrazines (L3pz), were synthesized and the related lanthanide complexes were studied in solution. The ligand containing three pyridylmethyl moieties L3py gives complexes with a coordination similar to EDTA, i.e. a hexadentate coordination mode as indicated by NMR and luminescence decays (q = 3) and stability constants in the range log β110 = 6.99–9.3 (La–Lu). On the other hand, the softest molecule L3pz forms much less stable complexes with log β110 = 4.0–4.4 (La–Eu). The selective back-extraction of Am(III) from organic solutions containing 4f and 5f elements was tested with the four water-soluble complexing agents. The ligand L3pz demonstrates poor stripping ability and selectivity. In contrast, the three ligands Lpy, Lpz and L3py give interesting back-extraction results with Eu/Am separation factors ranging from 36 to 46, which are significantly higher than with HEDTA. This exemplifies the role of the N-heterocycle softness in enhancing the separation between Am(III) and Eu(III). Interestingly, the pyrazine-based ligand, Lpz, demonstrates the best stripping properties, with a distribution factor that approaches that of HEDTA in the same conditions (DAm ≈ 0.3). This molecule is a good compromise between softness and hardness and forms complexes still stable at pH 3 due to its low basicity.

CH…O hydrogen bond in the crystal structure of a pyrazine-based ligand and determination of the amplitude of the ligand conformational change induced by Cu(II) coordination

The crystallographic structural features of the ditopic ligand pyrazine-2,5-dicarbaldehyde-bis(propionyl-N-methylhydrazone) 1, as well as their comparison with related ligands are herein reported. CH…O short contacts (2.46 and 2.55 Å) supporting network formation were observed. The amplitude of the conformational switch induced by the formation of the dinuclear rack-like complex [Cu 2 1(terpy) 2](CF 3SO 3) 4 of ligand 1 (synthesized by reaction of 2,5-pyrazine-dicarboxaldehyde with two equivalents of N-methyl-propionohydrazide), has been determined on the basis of the X-ray structures and it was found to be of 3 Å. Moreover, the correlation between the structural data and the data obtained by 2D NMR spectroscopy, is confirmed by the mean of the structural analysis herein discussed that concerns the protons involved in the NOE effects observed by NOESY spectroscopy.

Comparison of Two Tetrapodal N,O Ligands: Impact of the Softness of the Heterocyclic N-Donors Pyridine and Pyrazine on the Selectivity for Am(III) over Eu(III)

To quantify the impact of the N-donor softness on the coordination of f elements in aqueous solution, and in particular on the selectivity for Am(III) over Eu(III), we have designed the two tetrapodal hexadentate ligands N,N-bis(2-pyridylmethyl)ethylenediamine-N',N'-diacetic acid (Lpy) and N,N-bis(2-pyrazylmethyl)ethylenediamine-N',N'-diacetic acid (Lpz). These ligands bear two hard acetate groups to provide stability to the An(III) and Ln(III) complexes and two N-heterocyclic soft groups to provide Am(III) versus Eu(III) selectivity. They only differ in their N-donor moieties, pyridine or pyrazine. The proton NMR and potentiometric analyses performed on the lanthanide complexes of the two ligands indicate that a unique metallic complex, LnL, is formed and that LnLpy+ and LnLpz+ have the same structure in water. Furthermore, the hydration numbers of the europium and terbium ions in these complexes, measured by luminescence decay, have the same value (q = 3), indicating that the two ligands act as hexadentate donors in both systems. As expected, the softer pyrazine-based ligand gives less stable complexes than the pyridine-based ligand with the hard Ln(III) cations. The fragment N(CH2pz)2 containing two pyrazine functions has a very low contribution to the stability of the lanthanide complexes, even though the pyrazine groups are coordinated to the cation in water. The stabilities of the americium(III) complexes were determined by potentiometry and are greater than those found for the isoelectronic europium complexes. The selectivity for Am(III) over Eu(III) increases from 60 to 500 when the pyridine-containing fragment N(CH2py)2 is substituted by the pyrazine-containing fragment N(CH2pz)2, which demonstrates that the selectivity for Am(III) over Eu(III) is significantly enhanced when the softness of the N-heterocycle increases from pyridine to pyrazine. These new hydrophilic ligands present attractive selectivities for Am(III) over Eu(III) that could make them good candidates for the selective back extraction of Am(III) from organic solutions containing 4f and 5f elements.

Control of molecular architecture by use of the appropriate ligand isomer: a mononuclear "corner-type" versus a tetranuclear [2 x 2] grid-type cobalt(III) complex.

Employing two isomeric pyrazine-based ligands a [2 x 2] grid-type tetranuclear cobalt(III) complex, incorporating doubly deprotonated (La)2- ligands, and a "corner-type" mononuclear cobalt(III) complex, incorporating neutral H2Lp ligands in a zwitterionic form, have been synthesised and structurally characterised.

New chiral pyrazine-based ligands for self-assembly reactions

The syntheses of two families of ditopic N-heterocyclic ligands are described. All the compounds contain a central pyrazine ring connected to peripheral pyridine or bipyridine moieties, providing bipyridine- and terpyridine-like binding sites, respectively. Annellated terpene fragments render these metal chelators chiral. Thus they face the challenge of introducing stereoselectivity into the formation of chiral, multinuclear co-ordination species.

Palladium(II) Complexes of Pyridine- and Pyrazine-Based Ligands with Bis(carbon-metal) Bonds. Ligand Synthesis, Complexation, and Crystal Structure 1982

Palladium(II) Complexes of Pyridine- and Pyrazine-Based Ligands with Bis(carbon-metal) Bonds. Ligand Synthesis, Complexation, and Crystal Structure 1982

Chemistry of heterocyclic compounds. 81. Palladium(II) complexes of pyridine- and pyrazine-based ligands with trans bis(carbon-metal) bonds. Ligand synthesis, complexation, and crystal structure

Chemistry of heterocyclic compounds. 81. Palladium(II) complexes of pyridine- and pyrazine-based ligands with trans bis(carbon-metal) bonds. Ligand synthesis, complexation, and crystal structure