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PubMed2001

2-(Allylthio)pyrazine, a cancer chemopreventive agent, inhibits liver fibrosis induced by dimethylnitrosamine in rats: role of inhibition of transforming growth factor-beta1 expression.

Exposure to nitrosamines may be the occupational risk factor for liver cirrhosis. 2-(Allylthio)pyrazine, a chemopreventive agent, inhibits CYP2E1 and induces phase II enzymes. We examined the effects of 2-(allylthio)pyrazine on hepatic fibrosis, a prepathologic state of cirrhosis, and on the expression of transforming growth factor-beta1 induced by dimethylnitrosamine. Treatment of rats with dimethylnitrosamine for 4 weeks increased plasma alanine/aspartate amino-transferase and y-glutamyl transpeptidase activities, and bilirubin content, whereas the total plasma protein and albumin levels were decreased. 2-(Allylthio)pyrazine inhibited dimethylnitrosamine-induced increases in the enzyme activities and bilirubin, and restored the plasma protein and albumin contents. Masson's trichrome staining showed that dimethylnitrosamine induced liver fibrosis, the extent of which was reduced by 2-(allylthio)pyrazine treatments. Reverse transcription-polymerase chain reaction analysis revealed that 2-(allylthio)pyrazine inhibited production of transforming growth factor-beta1 mRNA by dimethylnitrosamine. These results demonstrated that 2-(allylthio)pyrazine might inhibit dimethylnitrosamine-induced liver fibrosis due to suppression of CYP2E1 expression and transforming growth factor-beta1 production.

Animals↗
PubMed1993

Effect of treatment with pyrazine and some derivatives on cytochrome P450 and some enzyme activities in rat liver.

1. The effect of pyrazine and three pyrazine derivatives, namely (methylthio) methylpyrazine (MTMP), 5, 6, 7, 8-tetrahydroquinoxaline (CHP) and 5-methyl-6, 7-dihydro-5'-cyclopentapyrazine (CPP), on hepatic peroxisomal and microsomal enzyme activities have been studied in male Sprague-Dawley rats. MTMP (0.25-2 mmol/kg per day) and the other compounds (1 mmol/kg/day) were administered by i.p. injections for 3 days. 2. None of the test compounds appeared to be peroxisome proliferators as there was no marked effect on hepatic palmitoyl-CoA oxidation, and neither pyrazine nor MTMP induced microsomal lauric acid 12-hydroxylase. 3. In contrast, all four compounds induced hepatic microsomal cytochrome P450-dependent enzyme activities. MTMP induced the metabolism of several mixed-function oxidase substrates including, 7-pentoxyresorufin, 7-benzoxyresorufin, benzphetamine, 4-nitrophenol and aniline, whereas pyrazine induced the metabolism of fewer substrates but including 4-nitrophenol and aniline. 4. By Western immunoblotting MTMP was found to increase levels of CYP2B1 and CYP3A isoenzymes, whereas pyrazine increased CYP2E1. 5. Thus, while pyrazine appears to be mainly a CYP2E inducer, MTMP is a mixed inducer of cytochrome P450 isoenzymes in the CYP2B, CYP3A and CYP2E subfamilies. CPP is probably a CYP2E inducer in rat liver, whereas CHP appears to be a mixed inducer of cytochrome P450 isoenzymes in the CYP2B, CYP3A and CYP2E subfamilies.

Animals↗
PubMed1983

Biological effects and structure-activity relationships of 1,2-dihydropyrido[3,4-b]pyrazines.

The effects of a number of 1,2-dihydropyrido[3,4-b]pyrazines (1-deaza-7,8-dihydropteridines) upon the proliferation and the mitotic index of cultured L1210 cells and upon the survival of mice bearing P388 leukemia were determined. The 1,2-dihydrostructure and amino groups or masked amino groups at positions 5 and 7 were necessary for activity, and various substituents at positions 2 and 3 had considerable influence upon the activity. A number of these pyrazines had significant activity against i.p. P388 leukemia in mice, and several pyrazines were more active than the corresponding oxazines or thiazines in both the in vitro and the in vivo systems. The effects of the pyrazines upon the cultured cells were reversible, and the rate and degree of reversibility were influenced by the substituents at positions 2 and 3. Tests performed with two of the pyrazines yielded results that indicate that these compounds, like the known agent nocodazole, might compete with colchicine for binding to tubulin. Synergistic killing of cultured L1210 cells was obtained with combinations of one of the pyrazines and vincristine.

Animals↗
PubMed1995

Pharmacological activities of imidazo[1,2-alpha]pyrazine derivatives.

The smooth muscle relaxant activity and other pharmacological properties of imidazo[1,2-alpha]pyrazine derivatives were compared with those of theophylline. Imidazo[1,2-alpha]pyrazine derivatives exhibited a potent smooth muscle relaxant activity regardless of the agent which had elicited the contraction and thus showed a broad spectrum of non specific smooth muscle relaxant activity. In the isolated guinea-pig atria, imidazo[1,2-alpha]pyrazine derivatives exhibited potent inotropic and chronotropic activities. As opposed to theophylline, the imidazo[1,2-alpha]pyrazine derivatives tested were unable to antagonize the adenosine-induced inhibition of spontaneous contractile activity of rabbit ileum. Furthermore, as opposed to theophylline, these derivatives did not exhibit a marked diuretic activity. Thus it appears that they do not act as adenosine receptor antagonists. Imidazo[1,2-alpha]pyrazine derivatives inhibited the total cAMP-phosphodiesterase (cAMP-PDE) and the total cGMP-phosphodiesterase (cGMP-PDE) activities of bovine trachea but with relatively low potencies, sharing a discrepancy between their activity on isolated tissues and their ability to inhibit PDE. It is suggested that imidazo[1,2-alpha]pyrazine derivatives may selectively inhibit type III and/or type IV phosphodiesterase isoenzymes involved in the regulation of the mechanical activity of cardiac and smooth muscle tissues.

Adenosine↗
PubMed2006

CASCSF study on the photochemical transposition reactions of pyrazines.

Several reaction pathways for the photochemical transformations of methyl-substituted pyrazine in its first excited state 1(pi --> pi*) have been determined using the CASSCF (six-orbital/six-electron active space) and MP2-CAS methods with the 6-311G(d) basis set. Our model investigations suggest that conical intersections play a crucial role in the photoisomerization of pyrazines. Moreover, the present theoretical findings indicate that all of the photoisomerizations of pyrazines adopt the same reaction path as follows: pyrazine --> Franck-Condon region --> conical intersection --> pyrimidine. That is, although an excited-state pyrazine molecule can initiate a phototransposition process easily, this process can be completed on the ground-state potential energy surface after passage through a conical intersection where a fast, radiationless decay is possible. The existence of these nonadiabatic reaction pathways is consistent with the available experimental observations of the photochemistry and photophysics of pyrazine and its methyl derivatives. In the present work, we propose a simple p-pi orbital topology model, which can be used as a diagnostic tool to predict the location of the conical intersections, as well as the geometries of the phototransposition products of various heterocycles.

Journal Article↗
PubMed1993

Pyrazine compounds and the measurement of cytosolic Ca2+.

Several of the pyrazine derivatives are widely used for inhibiting sodium flux via Na+/Ca2+ exchange or Na+/H+ exchangers or through the epithelial cation channels. These processes can profoundly affect cytosolic Ca2+. We found that the widely used fluorescent probes fura-2 and indo-1 could not be used to measure the effect of pyrazine analogs on the cytosolic free calcium ([Ca2+]i) of YAC-1 lymphoma cells treated with the pore-forming protein cytolysin/perforin. We show that the excitation spectra of pyrazine derivatives that specifically inhibit Na+/Ca2+ exchange [5-(N-4-chlorobenzyl)-2',4'-dimethylbenzamil], Na+/H+ exchange [5-(N-ethyl-N-isopropyl)-amiloride], and Na+ channels (phenamil) overlap with those of fura-2 and indo-1. In the presence of Ca2+, fluorescence readings for fura-2 plus drug are greater than those of fura-2 alone with the typically used 340- and 380-nm excitation light wavelengths; F380 readings were more affected than F340 readings. The effect was drug dose dependent. Hence, calculations that use F340 readings in the presence of pyrazine derivatives will result in overestimates of [Ca2+]i, while those that use the corresponding ratio readings, R340/380, will result in underestimates of [Ca2+]i. We found that the luminescent intracellular Ca2+ indicator aequorin could be used successfully with pyrazine derivatives, and that the ability of these compounds to enhance cytolysin/perforin-mediated increases in [Ca2+]i corresponded to their previously reported ability to inhibit Na+/Ca2+ exchange in pituitary cell plasma membrane vesicles. YAC-1 lymphoma cells are easy to culture and handle and may be a useful model for the studies of the Na+/Ca2+ exchanger in situ.

Aequorin↗
PubMed2000

2,3-dimethyl-5-(2-methylpropyl)pyrazine, a trail pheromone component of Eutetramorium mocquerysi Emery (1899) (Hymenoptera: Formicidae).

The ant Eutetramorium mocquerysi (Myrmicinae) is endemic to the island of Madagascar. During foraging and nest emigration the ants lay recruitment trails with secretions from the poison gland. We identified three pyrazine compounds in the poison gland secretion: 2,3-dimethyl-5-(2-methylpropyl)pyrazine 1, 2,3-dimethyl-5-(3-methylbutyl)pyrazine 3, 2,3-dimethyl-5-(2-methylbutyl)pyrazine 4. Only the first component elicited trail-following behavior in the ants. We were unable to investigate whether the other pyrazine components have a synergistic function.

Animals↗
PubMed1981

The effect of pyrazines on the metabolism of tryptophan and nicotinamide adenine dinucleotide in the rat. Evidence of the formation of a potent inhibitor of aminocarboxy-muconate-semialdehyde decarboxylase from pyrazinamide.

The intraperitoneal or oral administration of pyrazinamide and pyrazinoic acid (pyrazine 2-carboxylic acid) resulted in a marked increase of the NAD content in rat liver. The injections of pyrazine and pyrazine 2,3-dicarboxylic acid exhibited no significant effect on the hepatic NAD content. The boiled extract obtained from liver and kidney of rat injected with either pyrazinamide or pyrazinoic acid exhibited a potent inhibitory effect on the aminocarboxymuconate-semialdehyde decarboxylase (EC 4.1.1.45) activity in either lier or kidney, although pyrazinamide or pyrazinoic acid per se did not inhibit the enzyme activity. The unknown inhibitor of aminocarboxymuconate-semialdehyde decarboxylase was dialysable and heat-stable, and mostly excreted in urine by 6 and 12 h after injected of pyrazinoic acid and pyrazinamide, respectively. Pyrazine 2,3-dicarboxylic acid, pyrazine, nicotinamide, nicotinic acid, tryptophan, anthranilic acid, 5-hydroxyanthranilic acid and quinolinic acid exhibited no significant effect on the aminocarboxymuconate-semialdehyde decarboxylase activity in liver and kidney at the concentration of 1 mM in the reaction mixture. The expired 14CO2 from L-[benzen ring-U-14C]tryptophan was markedly decreased by the pyrazinamide injection, while the urinary excretion of 14C-labeled metabolites from L-tryptophan, mainly quinolinic acid, was markedly increased. These results suggest that the glutarate pathway of L-tryptophan was strongly inhibited by the inhibitor produced after the administration of pyrazinoic acid and pyrazinamide. Pyrazinamide but not pyrazinoic acid also exhibited a significant inhibition of the nuclear enzyme poly(ADP-ribose) synthetase in rat liver.

Animals↗
PubMed2006

Force field calculations and reassigments of Raman and IR frequencies of pyrazine-N,N'-dioxide.

Force field calculations have been carried out for the planar and non-planar modes of pyrazine-N,N'-O2 using the observed vibrational frequencies obtained from the IR and Raman spectral studies on pyrazine-N,N'-O2-h4 and pyrazine-N,N'-O2-d4 reported in the literature [D.A. Thornton, P.F.M. Verhoeven, G.M. Watkins, Herman O. Desseyn, Benjamin J. Van der Veken, Spectrochim. Acta 46A (1990) 1439]. The purpose of the present work is to determine force fields for the pyrazine-N,N'-O2 molecule and to present vibrational assignments for the observed IR and Raman frequencies to the fundamental modes, combination bands and overtones. The planar force field determined in the present case is expected to be better than that reported earlier [S. Szöke, G. Varsanyi, E. Baitz, Acta Chim. 53 (1967) 345] because of the inclusion of the observed frequencies due to pyrazine-N,N'-O2-d4 isotopomer. In addition, the non-planar force field for this molecule is reported for the first time.

Cyclic N-Oxides↗
PubMed1997

Apoptotic effects of imidazo[1,2-a]pyrazine derivatives in the human Dami cell line.

cAMP-elevating agents like phosphodiesterase inhibitors and purines have been shown to induce apoptosis. In the present work we have studied the effects of imidazo[1,2-a]pyrazine derivatives with a purine-like structure: PAB13 (6-bromo-8-(methylamino)imidazo[1,2-a] pyrazine), PAB15 (6-bromo-8-(ethylamino)imidazo[1,2-a]pyrazine), PAB23 (3-bromo-8-(methylamino)imidazo[1,2-a]pyrazine) on the growth of the Dami cell line in comparison to that of adenosine. The growth effect of PAB13, PAB15 and PAB23 was investigated in relation to their phosphodiesterase-inhibitory action and their activity on purinoceptors. Inhibition in cell growth was up to 71.0%, 76.3% and 89.7% for PAB23, PAB13 and PAB15, respectively and 100% for adenosine. Cell viability was affected in a concentration-dependent manner by PAB13, PAB15 and adenosine, with a correlation between growth inhibition and cytotoxicity. These effects of imidazo[1,2-a]pyrazine derivatives were found to be unrelated to an action on purinoceptors, but rather appear quantitatively linked to their ability in inducing apoptosis through their cAMP-increasing and phosphodiesterase-inhibitory potency.

Adenosine↗
PubMed2001

Formation of Strecker aldehydes and pyrazines in a fried potato model system.

Sugars and amino acids were removed from potato slices by soaking in water and ethanol. They were then infused with various combinations of sugars (glucose and/or fructose) and amino acids (asparagine, glutamine, leucine, isoleucine, phenylalanine, and/or methionine) and fried. Volatile compounds were trapped onto Tenax prior to gas chromatography-mass spectrometry. Relative amounts of compounds (relative to the internal standard) and relative yields (per mole of amino acid infused into the slices) were determined. Amounts of 10 pyrazines, 4 Strecker aldehydes, and 4 other compounds were monitored. Relative amounts and relative yields of compounds varied according to the composition of the system. For the single amino acid-glucose systems, leucine gave the highest relative amount and relative yield of its Strecker aldehyde. Asparagine and phenylalanine gave the highest total relative amount and total relative yield, respectively, of pyrazines. In the system containing all of the amino acids and glucose, the relative amount of 3-methylbutanal was higher, whereas the amounts of the other monitored Strecker aldehydes were lower. Most of the relative amounts of individual pyrazines were lower compared to the glucose-asparagine system, whereas the total relative yield of pyrazines was lower, compared to all of the single amino acid-glucose mixtures. Addition of fructose to the mixed amino acid-glucose model system generated Strecker aldehydes and pyrazines in ratios that were more similar to those of untreated potato chips than to those from the same system but without fructose. Both the sugars and the amino acids present in potato are crucial to the development of flavor compounds in fried potato slices.

Aldehydes↗
PubMed1999

Pyrazine formation from serine and threonine.

The formation of pyrazines from L-serine and L-threonine has been studied. L-Serine and L-threonine, either alone or combined, were heated at 120 degrees C as low temperature for 4 h or at 300 degrees C as high temperature for 7 min. The pyrazines formed from each reaction were identified by GC/MS, and the yields (to the amino acid used, as parts per million) were determined by GC/FID. It was found that pyrazine, methylpyrazine, ethylpyrazine, 2-ethyl-6-methylpyrazine, and 2,6-diethylpyrazine were formed from serine, whereas 2,5-dimethylpyrazine, 2,6-dimethylpyrazine, trimethylpyrazine, 2-ethyl-3,6-dimethylpyrazine, and 2-ethyl-3, 5-dimethylpyrazine were formed from threonine. Mechanistically, it is proposed that the thermal degradation of serine or threonine is composed of various complex reactions. Among these reactions, decarbonylation followed by dehydration is the main pathway to generate the alpha-aminocarbonyl intermediates leading to the formation of the main product, such as pyrazine from serine or 2, 5-dimethylpyrazine from threonine. Also, deamination after decarbonylation generates more reactive intermediates, alpha-hydroxycarbonyls. Furthermore, aldol condensation of these reactive intermediates provides alpha-dicarbonyls. Subsequently, these alpha-dicarbonyls react with the remaining serine or threonine by Strecker degradation to form additional alpha-aminocarbonyl intermediates, which then form additional pyrazines. In addition, decarboxylation and retroaldol reaction may also involve the generation of the intermediates.

Flame Ionization↗
PubMed1992

Synthesis and hypoglycemic activity of substituted 8-(1-piperazinyl)imidazo[1,2-a]pyrazines.

A series of alkyl- and halo-substituted 8-(1-piperazinyl)imidazo[1,2-a]pyrazines were prepared using two approaches, the condensation of alpha-halocarbonyl derivatives with an aminopyrazine or the oxidation-dehydration of a [(beta-hydroxyalkyl)amino]pyrazine. These imidazo[1,2-a]pyrazines were evaluated for their binding affinity to the alpha 1, alpha 2, beta 1, and beta 2 adrenergic receptors as well as their ability to lower blood glucose in insulin resistant hyperglycemic ob/ob mice. Modifications on 8-(1-piperazinyl)imidazo[1,2-a]pyrazine (4) reduced alpha 2 binding, lowered hypoglycemic potency, and showed variations in binding to the alpha 1, beta 1, and beta 2 adrenergic receptors. In addition to 4, the 2-methyl, 3-methyl, and 5-methyl 8-(1-piperazinyl)imidazo[1,2-a]pyrazines (16k, 25m, and 16f, respectively) displayed high affinity for the alpha 2 receptor and were potent hypoglycemic agents when compared to 2-amino-7,8-dihydro-4-(1-piperazinyl)-6H-thiopyrano[3,2- d]pyrimidine (MTP-1403, 2). Receptor binding was modified by use of a 4-methylpiperazine moiety which reduced alpha 1 and beta 1 binding while retaining some hypoglycemic activity. The structure-activity relationship for heterocyclic alkyl and halo substitution on biological activity is discussed.

Adrenergic alpha-Antagonists↗
PubMed2005

Prediction of supercritical ethane bulk solvent densities for pyrazine solvation shell average occupancy by 1, 2, 3, and 4 ethanes: combined experimental and ab initio approach.

We introduce a method that addresses the elusive local density at the solute in the highly compressible regime of a supercritical fluid. Experimentally, the red shift of the pyrazine n-pi electronic transition was measured at infinite dilution in supercritical ethane as a function of pressure from 0 to about 3000 psia at two temperatures, one close (35.0 degrees C) to the critical temperature and the other remote (55.0 degrees C). Computationally, stationary points were located on the potential surfaces for pyrazine and one, two, three, and four ethanes at the MP2/6-311++G(d,p) level. The vertical n-pi ((1)B(3u)) transition energies were computed for each of these geometries with a TDDFT/B3LYP/6-311++G(d,p) method. The combination of experiment and computation allows prediction of supercritical ethane bulk densities at which the pyrazine primary solvation shell contains an average of one, two, three, and four ethane molecules. These density predictions were achieved by graphical superposition of calculated shifts on the experimental shift versus density curves for 35.0 and 55.0 degrees C. Predicted densities are 0.0635, 0.0875, and 0.0915 g cm(-3) for average pyrazine primary solvation shell occupancy by one, two, and three ethanes at both 35.0 and 55.0 degrees C. Predicted densities are 0.129 and 0.150 g cm(-3) for occupancy by four ethanes at 35.0 and 55.0 degrees C, respectively. An alternative approach, designed to "average out" geometry specific shifts, is based on the relationship Deltanu = -23.9n cm(-1), where n = ethane number. Graphical treatment gives alternative predicted densities of 0.0490, 0.0844, and 0.120 g cm(-3) for average pyrazine primary solvation shell occupancy by one, two, and three ethanes at both 35.0 and 55.0 degrees C, and densities of 0.148 and 0.174 g cm(-3) for occupancy by four ethanes at 35.0 and 55.0 degrees C, respectively.

Computer Simulation↗
PubMed1997

[Reactions and syntheses of pyrazines].

This review deals with syntheses and reactions of pyrazines, especially, of 2,5-disubstituted pyrazines. The description was made in due to order of 1) synthesis and properties of 2,5-disubstituted pyrazines, 2) synthesis of 2,5-disubstituted pyrazine N-oxides, 3) synthesis of pyrazinols, 4) synthesis and utilization of pyrazinethiols; preparation of aldehydes, utilization as an acyl carrier, and preparation of olefins via the elimination of pyrazinylsulfinyl group, 5) synthesis of aminopyrazines, 6) synthesis of azidopyrazines and their transformation to imidazoles, 7) palladium-catalyzed reactions of chloropyrazines; dechlorination, introduction of cyano, alkenyl, alkynyl, alkyl, and aryl groups to the pyrazine ring. The cross-coupling of chloropyrazines with aromatic heterocycles such as furan, thiophene, pyrroles, indoles, benzo[b]furan, benzo[b]thiophene, oxazole, thiazole, benz[b]oxazole, and benzo[b]thiazole is also described.

Pyrazines↗
PubMedDate not supplied

Comparative study of antithrombotic and antiaggregatory activity of acetylsalicylic acid, ticlopidine and a new noncarboxylic acid antiinflammatory pyrazine derivative HF90.

The action of acetylsalicylic acid, ticlopidine and a new pyrazine derivative HF90 selected in preliminary screenings (11, 18, 19) was studied by using the mouse antithrombotic assay according to DiMinno and Silver (22) and in vitro blood platelet aggregation method according to Born (23). Acute pulmonary thromboembolism was induced by injection of a mixture of collagen and epinephrine into the mouse tail vein. The effect of HF90, an acidic pyrazine derivative possessing active methylene moiety, administered at doses of 50 and 100 mg/kg, was compared to the action of the well established antithrombotic agents: ticlopidine (100 mg/kg) and acetylsalicylic acid (20 mg/kg). The compounds were administered i.p. in single doses 1 h and 24 h before the thrombotic challenge or once a day per three consecutive days before the thrombotic challenge. Ticlopidine appeared to provide the better protection against microembolism than acetylsalicylic acid although its effect has not manifested itself immediately after administration. The pyrazine derivative examined has a lower but significant antithrombotic activity. The chemical class of pyrazine derivatives with active methylene moiety (the so called pyrazine CH/NH-acids) (16) provides a new original antiinflammatory pharmacophore and HF90 may serve as the "lead compound" in the search for new agents of pharmacological interest.

Animals↗
PubMed2004

Self-assembly of cuII and niII [2 x 2] grid complexes and a binuclear CuII complex with a new semiflexible substituted pyrazine ligand: multiple anion encapsulation and magnetic properties.

With the new substituted pyrazine ligand pyrazine-2,3-dicarboxylic acid bis[(pyridin-2-ylmethyl)amide], H(2)L, a binuclear complex [Cu(2)(LH)(Cl(3))(H(2)O)].H(2)O (1) and two [2 x 2]G grid complexes, [[Cu(4)(LH)(4)](ClO(4))(4)].5CH(3)OH.4H(2)O (2) and [[Ni(4)(LH)(4)]Cl(4)].5CH(3)CN.13H(2)O (3), have been synthesized and characterized spectroscopically and crystallographically. The ligand H(2)L crystallized in the triclinic space group P1, with a = 4.9882(7) A, b = 12.079(2) A, c = 14.454(2) A, alpha = 107.08(2) degrees, beta = 98.61(2) degrees, gamma = 97.54(2) degrees, V = 808.8(2) A(3), Z = 2, R1 = 0.0747, and R(w) = 0.1829 for 1319 observed reflections [I > 2 sigma(I)]. The molecule is L-shaped with a strong intramolecular bifurcated hydrogen bond in half of the molecule. In the crystal the molecules are linked by an intermolecular hydrogen bond to form a 1D polymer. The binuclear complex [Cu(2)(LH)(Cl(3))(H(2)O)].H(2)O (1) crystallized in the monoclinic space group P2(1)/a, with a = 8.6859(7) A, b = 28.060(2) A, c = 9.5334(9) A, beta = 107.89(1) degrees, V = 2211.2(3) A(3), Z = 4, R1 = 0.039, and R(w) = 0.097 for 1408 observed reflections [I > 2 sigma(I)]. There are two independent copper atoms both having square pyramidal geometry. Both coordinate to a pyrazine, a pyridine, and an amide N atom. Two chlorines complete the coordination sphere of one of the copper atoms, while one chlorine atom and a water molecule complete the coordination sphere of the other. The copper(II) [2 x 2] grid complex [[Cu(4)(LH)(4)](ClO(4))(4)].5CH(3)OH.4H(2)O (2) crystallized in the triclinic space group P1, with a = 17.1515(14) A, b = 17.7507(13) A, c = 19.3333(15) A, alpha = 67.34(1) degrees, beta = 69.79(1) degrees, gamma = 71.50(1) degrees, V = 4980.3(7) A(3), Z = 2, R1 = 0.083, and R(w) = 0.207 for 5532 observed reflections [I > 2 sigma(I)]. The four Cu(II) atoms are octahedrally coordinated by two pyrazine, two pyridine, and two amide N atoms and occupy the corners of a [2 x 2] grid with edge lengths, Cu...Cu, varying from 7.01 to 7.39 A. The nickel(II) [2 x 2] grid complex [[Ni(4)(LH)(4)]Cl(4)].5CH(3)CN.13H(2)O (3) crystallized in the monoclinic space group C2/c, with a = 16.3388(10) A, b = 29.754(2) A, c = 20.857(1) A, beta = 101.845(1) degrees, V = 9923.6(12) A(3), Z = 4, R1 = 0.050, and wR2 = 0.101 for 3391 observed reflections [I > 2 sigma(I)]. Here the complex possesses C(2) symmetry and again each metal atom is octahedrally coordinated to two pyrazine, two pyridine, and two amide N atoms. They occupy the corners of a [2 x 2] grid with an average edge length, Ni.Ni, of 6.97 A. Of the four anions (ClO(4)(-)'s in 2 and Cl(-)'s in 3) required to equilibrate the charges in the grid complexes, two are encapsulated, one above and one below the plane of the four metal atoms. The remaining two anions are located between the "wings" of the ligands. Magnetic susceptibility measurements indicate that the binuclear complex 1 is antiferromagnetic, with a J value of -15.07 cm(-1). This is larger than the J values found for the Cu(II) (2) and Ni(II) (3) grid complexes, which were -5.87 and -2.64 cm(-1), respectively. DFT calculations have been carried out to explain the difference in the J values found for complexes 1 and 2.

Journal Article↗
PubMed1998

A Proton-Induced N-1 to eta(2) Migration of the Fluxional Pyrazine in the [Ru(II)(hedta)(pz)](-) Complex.

[Ru(II)(hedta)(D(2)O)](-), hedta(3)(-) = N-(hydroxyethyl)ethylenediaminetriacetate, reacts with pyrazine in D(2)O at 25 degrees C to yield several isomers and </=20% of the pyrazine-bridged binuclear complex. Two isomers of 56.2% combined abundance have differentiated alpha (near) and beta (remote) (1)H NMR pyrazine resonances at 9.09 ppm (alpha or H2, H6 pair) and 8.33 ppm (beta or H3, H5 pair). The other isomer of ca. 24% abundance exhibits only a singlet at 8.76 ppm, indicative of fluxional pyrazine movement from N-1 to N-4. This is believed to be the cis-polar isomer. Within 24 h the differentiated isomers convert to the fluxional isomer, which remains fluxional in 50% D(2)O/50% CD(3)OD down to 237 K. The fluxional isomer has all equivalent (13)C NMR resonances at 152.44 ppm, 5.18 ppm downfield of free pyrazine. Comparison with the bridged binuclear ion {[Ru(hedta)](2)(pz)}(2)(-) revealed fortuitously similar shifts; the (1)H NMR spectrum shows a singlet at 8.76 ppm, and the (13)C NMR spectrum, a singlet at 152.40 ppm. These species have different electrochemical signatures, however, with the 1:1 fluxional complex having a Ru(II/III) wave at 0.20 V that shifts to 0.35 V upon protonation of the N-4 position, whereas the binuclear complex has two waves at 0.18 and 0.33 V which are independent of pH. (1)H NMR indicates stereochemically rigid coordination of 2-methylpyrazine (2-CH(3)pz) at N-4 with the 2-CH(3) position remote in the major species (65.5%) and at N-2 with the 2-CH(3) site adjacent (34.4%) in the lesser isomer. The proton resonances are as follows. Remote isomer (N-4): H2, 8.22 ppm; H3, 8.97 ppm; H5, 8.88 ppm; CH(3), 2.53 ppm. Adjacent (N-1) isomer: H2, 8.73 ppm; H3, 8.42 ppm, H5, 8.50 ppm; CH(3), 2.45 ppm. A slow conversion of the strained adjacent isomer to the remote isomer is observed. Remote and adjacent isomers were also prepared for [(NH(3))(5)Ru(2-CH(3)pz)](2+) in 87.7% and 12.3% yield. Protonation of [Ru(hedta)(pz)](-) yields nonfluxional complexes: a major species (63%) bound eta(2)(1,2) with four complicated resonance patterns at 8.72, 8.57, 8.00, and 7.91 ppm, each signal having across-the-ring couplings which require couplings to at least two different ring protons and a minor N-1-coordinated N-4 protonated species (16%) with alpha and beta proton pairs resonating at 8.81 and 8.25 ppm. An eta(2)(2,3) isomer is also detectable (20%) which has four types of (1)H resonances at 9.51, 9.01, 8.85, and 8.15 ppm. The weakened sigma bonding from pzH(+) and enhanced pi-acceptor capacity power of pzH(+) combine to induce a switch in coordination to either eta(2)(1,2) or eta(2)(2,3).

Journal Article↗