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Two new 3D NaCl-type frameworks of [M(pyrazine)2NO2]ClO4 (M = Co, Cu), consist of (4, 4) sheets of metal-pyrazine connected by mu(1,3)-nitrito, representing the first examples of mu(1,3)-nitrito bridges and exhibiting antiferromagnetism.
Density Functional Theory (DFT) at the generalized gradient approximation (GGA) level has been applied to the complexes [Fe(CN)(5)L](n-) and [Ru(CN)(5)L](n-) (L = pyridine, pyrazine, N-methylpyrazinium), as well as to [Fe(CN)(5)](3)(-) and [Ru(CN)(5)](3)(-). Full geometry optimizations have been performed in all cases. The geometrical parameters are in good agreement with available information for related systems. The role of the M(II)-L back-bonding was investigated by means of a L and cyanide Mulliken population analysis. For both Fe(II) and Ru(II) complexes the metal-L dissociation energies follow the ordering pyridine < pyrazine < N-methyl pyrazinium, consistent with the predicted sigma-donating and pi-accepting abilities of the L ligands. Also, the computed metal-L bond dissociation energies are systematically smaller in the Ru(II) than in the Fe(II) complexes. This fact suggests that previous interpretations of kinetic data, showing that ruthenium complexes in aqueous solution are more inert than their iron analogues, are not related to a stronger Ru-L bond but are probably due to solvation effects.
Reactions of Cu(hfacac)(2).H(2)O (hfacac = hexafluoroacetylacetonate) with pyrazine, 3-cyanopyridine, 1,2-bis(4-pyridyl)ethane, and 4,4'-trimethylenebipyridine in methylene chloride have been studied. Two types of products were obtained and crystallographically characterized: compounds 1 (Cu(hfacac)(2)(pyrazine)(2), orthorhombic, Pbca; a = 9.839(2) Å, b = 17.984(2) Å, c = 13.572(2) Å, Z = 4) and 2 (Cu(hfacac)(2)(3-cyanopyridine)(2), triclinic, P&onemacr;, a = 13.175(2) Å, b = 13.955(2) Å, c = 7.936(2) Å, alpha = 104.29(2) degrees, beta = 105.90 degrees, gamma = 81.47(1) degrees, Z = 2) were isolated as molecular complexes, while compounds 3 (Cu(hfacac)(2)(1,2-bis(4-pyridyl)ethane, monoclinic, P2(1)/c, a = 7.922(1) Å, b = 16.266(4) Å, c = 10.163(2) Å, beta =93.83(1) degrees, Z = 2) and 4 (Cu(hfacac)(2)(4,4'-trimethylenebipyridine), monoclinic, Pc, a = 7.940(1) Å, b = 9.691(2) Å, c = 18.539(3) Å, beta = 95.83(2) degrees, Z = 2) were isolated as one-dimensional infinite polymeric chains. The structures were further characterized by infrared spectroscopy, thermogravimetry, and elemental analysis; additionally, the magnetic behavior of all compounds was investigated and found to follow the Curie law. All four compounds feature similar 4 + 2 pseudooctahedral coordination environments around the Cu(II) centers composed of square-planar Cu(hfacac)(2) units with the N-donor ligands occupying the trans positions. The length of the N-donor ligands determines the formation of either molecular complexes (1 and 2, short ligands) or infinite polymeric structures (3 and 4, longer ligands). Important C-F.H-C hydrogen-bonding interactions in compounds 1 and 3 play a significant role in aligning the molecules and the polymer strands in the crystalline solids.
Preparations of a series of face-capped octahedral hexarhenium(III) clusters having two N-heterocyclic ligands, [Bu4N]2[trans-[Re6(mu 3-S)8Cl4(L)2]] (Bu4N+ = tetra-n-butylammonium cation; L = pyrazine (1a), 4,4'-bipyridine (3a), 4-methylpyridine (5a), 4-(dimethylamino)pyridine (6a)) and their cis analogues (1b, 3b, 5b, and 6b, respectively), and their electrochemical and photophysical properties have been reported. An X-ray crystal structure determination has been carried out for 1a to confirm the trans configuration (C40H80N6S8Cl4Re6, orthorhombic, space group Cmca (No. 64), a = 19.560(5) A, b = 19.494(4) A, c = 18.592(4) A, beta = 115.76(2) degrees, Z = 4). The redox potential of the reversible ReIII6/ReIII5ReIV process of these complexes and previously reported [Bu4N]2[trans- and cis-[Re6(mu 3-S)8Cl4(4-cyanopyridine)2]] (2a and 2b, respectively) and [Bu4N]2[trans- and cis-[Re6(mu 3-S)8Cl4(pyridine)2]] (4a and 4b, respectively) in acetonitrile depends linearly on the pKa of the N-heterocyclic ligands, with the potentials being more negative with basic ligands. The ligand-centered-redox waves for 1a, 1b, 2a, and 2b were observed as split waves (delta E1/2 = 90-140 mV), the extent of the splitting being larger for the cis isomer and largest for the pyrazine complexes. Electronic interaction between the two ligands through the [Re6(mu 3-S)8]2+ core has been suggested. The second ligand-reduction wave was also observed for 3a and 3b, the potential being shifted positively to coalesce with the first reduction wave on addition of the weak proton donor imidazole. This is accounted for by the proton-coupled redox reaction at the free pyridyl site of the 4,4'-bipyridine ligands. All of the complexes show luminescence in acetonitrile at room temperature. While the complexes of pyridine and 4-methylpyridine show photophysical characteristics (lambda em 740-750 nm, phi em 0.031-0.057, tau em 4.2-6.2 microseconds) similar to those (770 nm, 0.039, and 6.3 microseconds, respectively) of [Re6(mu 3-S)8Cl6]4-, emissions of other complexes are significantly weak with lambda em, phi em, and tau em values in the ranges 763-785 nm, 0.0010-0.0017, and 0.013-0.029 microsecond, respectively. Suggestions are given for the excited states localized on the cluster core and the ligand pi* orbitals.
We have used quasielastic neutron scattering to probe the solid-state ligand dynamics in the coordination polymer Mn[N(CN)(2)](2)(pyz) [pyz = pyrazine] which has double-interpenetrating 3D lattices. A reversible structural phase transition occurs at 410 K as shown by neutron spectroscopy and differential scanning calorimetry. The origin of this transition is linked to rotational dynamics associated with the bridging pyz ligands. At 425 K, the pyrazine ring motion can be solely regarded as a 180 degrees reorientational jump about the axis defined by the Mn-N coordinative bonds, occurring with a correlation time of approximately 70 ps. This model can be extended to the 200-410 K temperature region using high-resolution backscattering spectroscopy to measure an identical motion on the time scale of nanoseconds with an activation energy of 24 +/- 2 kJ mol(-1). In contrast, no quasielastic scattering is seen for the 2D layered variant beta-Cu[N(CN)(2)](2)(pyz), owing to its more compact layer packing motif. Importantly, this work represents the very first study of solid-state rotational dynamics in an interpenetrating lattice structure.
The ethyl (1,2-dihydropyrido[3,4-b]pyrazin-7-yl)carbamates have been reported to bind with cellular tubulin, to produce an accumulation of cells at mitosis, and to exhibit cytotoxic activity against experimental neoplasms in mice. Studies on the disposition of ethyl (5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazin-7 -yl)carbamate (8) in mice showed that one metabolite was formed by cleavage of the ethyl carbamate moiety. Analogues with alterations in the carbamate group were prepared by transformations at the carbamate of 8, by reductive cyclization of nitropyridine intermediates, and by hydride reduction of the ring of heteroaromatic compounds. In vitro and in vivo evaluations of analogues indicated that a carbamate group was required for activity. No significant change in activity was observed when ethyl was replaced by methyl. However, activity was reduced when ethyl was replaced with bulky aliphatic groups and when ethoxy was replaced with a methylamino group. Also, the activity of 8 was decreased by acetylation of the 5-amino group and was destroyed by substitution of an amino group at the 8-position.
A general synthetic route has been developed for the efficient preparation of 2,3-disubstituted thieno[3,4-b]pyrazines. These methods eliminate problems in the preparation of the precursor 3,4-diaminothiophene and utilize alpha-diones prepared through the reaction of the appropriate organocuprates with oxalyl chloride. This combination allows the convenient preparation of thieno[3,4-b]pyrazine and its 2,3-disubstituted analogues (where substituent = methyl, hexyl, octyl, decyl, dodecyl, and phenyl) in high yield. Characterization of the structure and reactivity of this class of compounds is also described, including the results of structural, electrochemical, and pK(a) studies.
[reaction: see text] The one-pot synthesis of new 9-alkyl-6-chloropyrido[3,2-e][1,2,4]triazolo[4,3-a]pyrazines has been achieved. Hydrazides regioselectively reacted as nucleophiles with the 3-chloro substituent of 2,3-dichloropyrido[2,3-b]pyrazine. An intramolecular cyclization afforded the tricycle nonxanthine adenosine receptor antagonists.
Classical trajectory calculations were performed to simulate state-resolved energy transfer experiments of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) and CO(2), which were conducted using a high-resolution transient infrared absorption spectrometer. The goal here is to use classical trajectories to simulate the supercollision energy transfer pathway wherein large amounts of energy are transferred in single collisions in order to compare with experimental results. In the trajectory calculations, Newton's laws of motion are used for the molecular motion, isolated molecules are treated as collections of harmonic oscillators, and intermolecular potentials are formed by pairwise Lennard-Jones potentials. The calculations qualitatively reproduce the observed energy partitioning in the scattered CO(2) molecules and show that the relative partitioning between bath rotation and translation is dependent on the moment of inertia of the bath molecule. The simulations show that the low-frequency modes of the vibrationally excited pyrazine contribute most to the strong collisions. The majority of collisions lead to small DeltaE values and primarily involve single encounters between the energy donor and acceptor. The large DeltaE exchanges result from both single impulsive encounters and chattering collisions that involve multiple encounters.
[reaction: see text] A novel route to highly substituted chiral 6,7-dihydro-5H-imidazo[1,5-a]pyrazine-8-ones starting from Meldrum's acid is disclosed. The key features of the methodology are the incorporation of amino esters as a chiral pool and facile mild intramolecular cyclization to form the pyrazine ring. Incorporation of various substituents at different stages of the synthesis from various building block sets makes this methodology readily amenable to parallel synthesis.
The covalent binding of pyrazine on Si(100) have been investigated using high-resolution electron energy loss spectroscopy (HREELS) and x-ray photoelectron spectroscopy. Experimental results clearly suggest that the attachment occurs exclusively through the bonding of the two para-nitrogen atoms with the surface without the involvement of the carbon atoms, as evidenced from the retention of the (sp2) C-H stretching mode in HREELS and a significant down shift of 1.6 eV in the binding energy of N 1s. The binding mechanism for pyrazine on Si(100) demonstrates that reaction channels for heteroatomic aromatic molecules are strongly dependent on the electronic properties of the constituent atoms.
Supercollision relaxation of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) with D35Cl is investigated using high-resolution transient IR diode laser absorption spectroscopy at 4.4 microm. Highly excited pyrazine is prepared by pulsed UV excitation at 266 nm, followed by rapid radiationless decay to the ground electronic state. The rotational energy distribution of the scattered DCl (v = 0,J) molecules with J = 15-21 is characterized by T(rot) = 755+/-90 K. The relative translational energy increases as a function of rotational quantum number for DCl with T(rel) = 710+/-190 K for J = 15 and T(rel) = 1270+/-240 K for J = 21. The average change in recoil velocity correlates with the change in rotational angular momentum quantum number and highlights the role of angular momentum in energy gain partitioning. The integrated energy-transfer rate for appearance of DCl (v = 0,J = 15-21) is k(2)(int) = 7.1x10(-11) cm3 molecule(-1) s(-1), approximately one-eighth the Lennard-Jones collision rate. The results are compared to earlier energy gain measurements of CO2 and H2O.
An efficient QP partitioning algorithm to compute the eigenvalues, eigenvectors, and the dynamics of large molecular systems of a particular type is presented. Compared to straightforward diagonalization, the algorithm displays favorable scaling (proportional to N(T)2) as a function of N(T), the size of the Hamiltonian matrix. In addition, the algorithm is trivially parallelizable, necessitating no "cross-talk" between nodes, thus enjoying the full linear speedup of parallelization. Moreover, the method requires very modest storage space, even for extremely large matrices. The method has also been enhanced through the development of a coarse-grained approximation, enabling an increase of the basis set size to unprecedented levels (10(8)-10(10) in the current application). The QP algorithm is applied to the dynamics of electronic internal conversion in a 24 vibrational-mode model of pyrazine. A performance comparison with other dynamical methods is presented, along with results for the decay dynamics of pyrazine and a discussion of resonance line shapes.
Quantum control of the S(2)-->S(1) internal conversion in a complete 24-mode dimensionality model of pyrazine is demonstrated. The fully quantum mechanical study makes use of the recently developed "QP algorithm" for performing accurate computations of projected quantum dynamics and the role of overlapping resonances in control. The results are extremely encouraging, demonstrating active control over internal conversion so as to almost completely suppress the process over time scales of approximately 50-100 fs [well in excess of the natural internal conversion times (approximately 20 fs)] or to accelerate it to complete internal conversion in less than 5 fs. A number of new diagnostics are introduced to demonstrate the significance of overlapping-resonance contributions to control. Control is far better than for a reduced dimensionality model of pyrazine, presumably because of the increased degree of overlap between bound state resonances existing in the full dimensionality case.
We investigate the molecular dynamics of pyrazine after excitation to the S2 electronic state by using the time-dependent discrete variable representation (TDDVR) method. The investigation has been carried out with a realistic 24-mode model Hamiltonian consisting of all the vibrational degrees of freedom of pyrazine molecule. First, we perform the simulation on a basic four-mode model, and then by including additional eight important modes and finally, by introducing 20 bath modes on the basic model. This sequential inclusion of bath modes demonstrates the effect of weak modes on the subsystem, where the calculations of energy and population transfer from basic model to the bath quantify the same effect. The spectral profile obtained by using TDDVR approach shows reasonably good agreement with the results calculated by quantum mechanical approach. It appears that the TDDVR approach for those large systems where quantum mechanical description is needed in a restricted region is a good compromise between accuracy and speed.
The new nickel(II) coordination polymer poly[diaquanickel(II)-mu-(pyrazine-2,3,5,6-tetracarboxylato)-tetraaquanickel(II)], [[[Ni(C(8)N(2)O(8))(H(2)O)(2)]Ni(H(2)O)(4)]](n), has been synthesized and characterized both spectroscopically and crystallographically, by X-ray powder diffraction analysis. In this two-dimensional coordination polymer, Ni(II) ions are bridged by pyrazine-2,3,5,6-tetracarboxylic acid, coordinating in a bis-bidentate manner, so forming one-dimensional polymeric chains. The chains are linked by a second Ni(II) ion, via an O atom of the coordinated carboxylate group, resulting in the formation of a two-dimensional layer-like polymer. The remaining coordination sites of the two independent octahedral Ni(II) ions are occupied by water molecules. The layers are connected via hydrogen bonds involving all six coordinated water molecules.
5,6-Bis(2-pyridyl)pyrazine-2,3-dicarboxylic acid exists as an inner-salt zwitterion, 3-carboxy-5-(2-pyridinio)-6-(2-pyridyl)pyrazine-2-carboxylate, (Ia), C(16)H(10)N(4)O(4). The adjacent pyridine and pyridinium rings are almost coplanar due to the presence of an intramolecular hydrogen bond involving the pyridine N atom and the NH H atom of the pyridinium group. In the crystal of (Ia), symmetry-related molecules are hydrogen bonded via the carboxylic acid OH group and one of the carboxylate O atoms to form a polymer, which exhibits a channel-type structure. In the HCl, HClO(4) and HPF(6) salts, 6-carboxy-5-carboxylatopyrazine-2,3-diyldi-2-pyridinium chloride 2.25-hydrate, (II), C(16)H(11)N(4)O(4)(+).Cl(-).2.25H(2)O, 6-carboxy-5-carboxylatopyrazine-2,3-diyldi-2-pyridinium perchlorate trihydrate, (IIIa), C(16)H(11)N(4)O(4)(+).ClO(4)(-).3H(2)O, and 6-carboxy-5-carboxylatopyrazine-2,3-diyldi-2-pyridinium hexafluorophosphate trihydrate, (IIIb), C(16)H(11)N(4)O(4)(+).PF(6)(-).3H(2)O, both pyridine rings are protonated. In the perchlorate form, and in the isomorphous hexafluorophosphate form, the molecule possesses C(2) symmetry, with has a symmetrical intramolecular hydrogen bond involving the adjacent carboxylate and carboxylic acid substituents. In the crystals of the chloride and perchlorate (or hexafluorophosphate) salts, hydrogen-bonded polymers are formed which are three-dimensional and one-dimensional, respectively.
The title compound, [Cu8(C8H24O2Si)2(C3H7NO)8].C4H4N2.C3H7NO, features a sandwich-like cage enclosing a pyrazine molecule, both situated on a centre of inversion. In addition, the crystal structure contains one dimethylformamide molecule which is disordered over a centre of inversion. The copper layer, containing eight atoms, is located between two siloxanolate fragments. The whole structure of Cu atoms and siloxanolate rings is distorted by the pyrazine molecule, leading to an oval form. As a result, the angles between the Cu atoms differ at the copper layer. The difference in the angles could lead to some deviations in the Cu-Cu exchange interactions within the copper ring, which is of interest for molecular magnetism.