Diazines structure

With this in mind, the coordination chemistry of 52 with different diazine structural isomers was investigated. There were no detectable changes in the H NMR spectrum of 52 in a THF-Jg solution when either pyrazine or pyrimidine were added in 1 1 or 1 2 molar ratios, which suggested that only weak interactions might occur between 52 and these bases. In contrast, incremental addition of pyridazine or phthalazine to a THF-Jg solution of 52 at 25 °C resulted in an upheld shift of the aromatic NMR resonances of the diindacycle 52 thus reflecting the formation of complexes between 52 and the 1,2-diazines. Analysis of the tritration data clearly indicated the formation of 1 1 Lewis acid-diazine complexes 52-pyridazine-(THF)2 and 52-phthalazine-(THF)2 whose stability constants are equal to 80 ( 10) and 1000 ( 150) M respectively (Scheme 29). These data, as a whole, indicate that 52 is a selective receptor for 1,2-diazines. 

With Hg2 bidentate nitrogen ligands form complexes that have chain structures. For example, 1,4-diazine III forms a complex [Hg2(C4H4N2XN03)2] (IV) that has Hg—Hg units joined in infinite chains, with the [N03] groups between the chains . ... 

Isolated and benzo-fused diazine rings are key structural elements in many natural and synthetic compounds of current interest. This contribution relates highlights from many of the studies on the diazines pyridazine, pyrimidine, pyrazine, and their benzo-fused derivatives cinnoline, phthalazine, quinazoline, quinoxaline, and phenazine published in English in the journal literature during 1996, as covered by Chem. Abstr. through volume 126, issue 5. 

Second only to sulfur-based systems, nitrogen complexes are relatively well represented in the structural literature with 41 complexes reported. Of these, 25 are with I2 as the electron acceptor, 11 are with the interhalogen IC1, three are with Br2, and two are with IBr. As expected, in every case the halogen bond forms between the nitrogen and the softest halogen atom, i.e., iodine, in all of the complexes except those with dibromine. Most N I2 complexes, and all N Br2, N IBr, and N IC1 complexes are simple adducts, mode A. Exceptions for the diiodine complexes include bridging mode (B) observed for diazines, such as pyrazine [86], tetramethylpyrazine [86], phenazine, and quinoxaline [87], and for 9-chloroacridine [89] and the 1 1 complex of diiodine with hexamethylenetetramine [144] and amphoteric bridging mode (BA) observed for 2,2 -bipyridine [85], acridine [89], 9-chloroacridine [89], and 2,3,5,6-tetra-2/-pyridylpyrazine [91]. The occurrence of both B and BA complexes with 9-chloroacridine, and of B and A complexes and an... 

The copper(I) analogue [Cu(L)][C104] (758) (distorted pseudotetrahedral) of complex (294) was prepared and its properties investigated.258 Using a bis(diazine) heterocyclic ligand structure of the complex (759) was determined. 64... 

The Boger pyrrole synthesis based on a heterocyclic azadiene Diels-Alder strategy (1,2,4,5-tetrazine to 2,2-diazine to pyrrole) was employed by the author for the total synthesis of ningalin B . Thus a Diels-Alder reaction of the electron-rich acetylene 52 with the electron deficient 1,2,4,5-tetrazine 53 proceeded to give the desired diazine 54 which underwent subsequent ring contraction to afford the core pyrrole structure 55. 

Problem 20.43 Write structures for (a) oxirane, (b) 1,2-oxazole, (c) 1,4-diazine (pyrazine), (d) l-thia-4-oxa-6-azocine, (e) 3H-1,2,4-triazole, (/) azepane. 

Two positional isomers of pyrimidine are pyridiazine and pyrazine, which only differ structurally from pyrimidine in terms of the position of the nitrogen atoms in the ring. These three heterocycles together with their derivatives are known as diazines. 

Pyridazine, one of the three possible isomeric diazines, is assumed to be a planar six-membered ring and is represented as a resonance hybrid of two structures (la) and (lb) with a greater contribution from the canonical structure (la). 

The first coordination compound of this type was the aniline complex Hg(PhNH2)2(N03)2,96 but later research revealed that up to six ligands are coordinated to Hg2(N03)2. The first X-ray crystal structure determination of this type of coordination compound was published in 1967 for Hg2(phen)(N03)2.98 In this complex one Hg atom of the Hg+—Hg+ ion is bonded to both N atoms of 1,10-phenanthroline the other Hg atom is coordinated to oxygen atoms of the nitrate ions.98 More recently X-ray structure determinations have been made with complexes of Hg2(N03)2 with either 1,4-diazine (10)99 or quinoline (11).100... 

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