Molecular and electronic structures and conformational analysis of derivatives of 9-nitroanthracene, anionic σ-complexes

Abstract

Potassium 10-methoxy-9-nitroanthracenide was studied by X-ray diffraction analysis. The central ring of the anthracene fragment adopts an asymmetrically flattened boat conformation. The bond lengths in the C(Ar)--C(NO2)--C(Ar) fragment are indicative of the presence of conjugation between the 7t-systems of the benzene rings and the nitro group owing to a substantial contribution of the aci form to the structure of the nitro group.Quantum-chemical calculations of anionic cr-complexes of 9-nitroanthracene derivatives were performed. In all cases, the central ring adopts two conformations with the pseudoaxial and pseudoequatoria[ orientations of the substituents at the saturated C(10) atom, respectively. The relative stability of the conformers and the factors determining the stability were considered. The minima on the potential energy surface have a flattened shape in spite of relatively high barriers to the conformational transition (2--4 kcal tool-l). The deviation of the C(Ar)--C(Ar)--C(sp3)--C(Ar) torsion angle from the equilibrium value by +20 ~ causes an increase in the energy of the anion by less than 1 kcal tool -I. The effect of the substituents oll the charge distribution was considered.Key words: 9-nitroanthracene, anionic <~-complexes, molecular structure, conformational analysis, X-ray diffraction analysis, semiempirical quantum-chemical AM1 method.Anionic c~-complexes of aromatic polynitro compounds are intermediates in reactions of nucleophilic substitution, l Studies of the spatial structures of these compounds can provide important data on the reactivity and fine details of the reaction mechanisms. Unfortunately, because of the instability of or-complexes, it is generally not only difficult to study them by X-ray diffraction analysis but even impossible to isolate these compounds from solutions. Because of this, experimental data z-s on the structures of these compounds are scarce.The aim of this work is to theoretically and experimentally study the molecular and electronic structures of anionic G-complexes of 9-nitroanthracene 1--3. 2 3Experimental Potassium salt of anion 2 was prepared according to a procedure reported previously. 9 Crystals were grown by slow cooling of a solution of the salt in methanol.X-ray diffraction analysis. Crystals of the salt 2-K +-5MeOH-3H20 are monoclinic. At -80 ~ a = 21.398(8) A, b = 7.564(3) A, c = 13.295(5) A, 13 = 107.21(3)% V = 2056(3) A 3, space group C2, Z = 4, de.ale = 1.342 gcm -3, /~ = 0.295 mm -t. Intensities of 1900 independent reflections (Pin ~ = 0.08) were measured on a four-circle automated Syntex P21/PC diffractometer (graphite monoehromator, Mo-Kct radiation, 0/20 scanning technique, 20max = 50*). We carried out the profile analysis of the X-ray diffraction data according to a known procedure, 10 which made it possible to improve substantially the quality of the data set.The structure was solved by the direct method with the use of the SHELXTL PLUS 4.2 program package. 11 The positions of the hydrogen atoms of the anthracene fragment were calculat...

X-ray Diffraction and Ab Initio Quantum-Chemical Study of the Charge Density in the Crystals of Meisenheimer ComplexesDerivatives of 2,4,6-Trinitrobenzene and 5,7-Dinitroquinoline

“…The C(5)−N(2) bond 1.399(1) Å is shorter than the C(7)−N(3) bond 1.414(1) Å. It should be noted as well that both C−NO 2 bonds are significantly shorter in the anion 2 as compared to the anionic σ-complex 1 (see also 9,43 ), but are much longer than the C−N bond (1.36(2) Å) in the Meisenheimer complex of 9-nitroanthracene . This indicates an essential increase of the partially double character of the C−NO 2 bonds with expansion of the aromatic π-system.…”

“…Previous theoretical studies of the MC demonstrated some contradictions between results of calculations using different semiempirical ,, and ab initio ,, methods as well. For example, the study of the nucleophilic aromatic substitution reaction between 4-Cl−Ph-CO−SMe and CH 3 COO - by the PM3 and DFT methods indicated a formation of the stable MC during reaction.…”

“…Probably, structural investigations of spirocyclic anionic σ-complexes are more promising in this respect because these MCs were found to form relatively good and stable single crystals. Our recent studies of MCs were devoted to the conformational analysis of similar spirocyclic compounds, , or the study of zwitterionic Meisenheimer complexes. − Moreover, we have investigated, for the first time, the charge density distribution in the crystal of the Meisenheimer complex of the potassium 3-methyl-5‘,7‘-dinitro-5‘,8‘-dihydrospiro(1,3-oxazolidine-2,8‘-quinolinide) . This preliminary study has demonstrated that the electronic structure of the anionic σ-complex is drastically influenced by the manner of potassium cation coordination to the organic anion.…”