N. P. Gambaryan

Crown compounds for anions. A new approach to the description of chemical bonds in the complexes of halide anions with polymercury-containing macrocycles

The nature of chemical bonds between halide anions and polymercury-containing macrocycles has been investigated. For this purpose, calculations of the complexes of halide anions with cyclic trimeric o-phenylenemercury (o-C6H4Hg)3, cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4Hg)3, cyclic pentameric perfluoroisopropylidenemercury [(CF3)2CHg]5 and some of their simpler hypothetical analogs have been carried out by the MNDO method. Complexes of the half-sandwich [L—X]−, bipyramidal [X—L—X]2− and sandwich [L—X—L]− (X = Hal, L = mercury-containing macrocycle) types have been calculated. It has been established that a uniform description of the bonding in such complexes, not depending on the number of Hg atoms in the macrocycle, can be reached within the framework of the concept of generalized chemical bonds.

Crown compounds for anions. Spectroscopic and theoretical studies of complexation of borohydride anions with cyclic trimeric perfluoro-o-phenylenemercury

It has been shown by IR and NMR spectroscopy that cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4Hg)3 (1) is capable of binding borohydride anions in THF and 1,2-dichloroethane solutions to form complexes {[(o-C6F4Hg)3](BH4)2}2− (2) and {[(o-C6F4Hg)3]2(BH4)}− (3). According to the IR data, complex 2 contains both terminal and coordinated B–H bonds while all four B–H bonds of the BH4− ion are coordinated with the mercury atoms in complex 3. The use of NMR spectroscopy provided the identification of one more complex of 1 with BH4−, which can be formulated as {[(o-C6F4Hg)3](BH4)}− (4). The stability constants of complexes 2 and 3 have been determined by IR spectroscopy. Quantum-chemical calculations of 2–4 by the AM1 method suggest that the complexes have the unusual bipyramidal, sandwich and half-sandwich structures, respectively. In contrast to 1, the interaction of (C6F5)2Hg with BH4− anions in THF gives a single complex {[(C6F5)2Hg](BH4)}− (5) independently of the reagent ratio. This complex is much less stable than 2 and 3.

Fullerene C{sub 60} as a {eta}{sup 5-} and {eta}{sup 6}-ligand in sandwitch-type {pi}-complexes with transition metals

The molecular and electronic structures of some hypothetical sandwich-type π-complexes of transition metals with fullerene C60 were modeled. The M-C60 bonds in η5-C60MCp+ complexes (M = Fe, Ru, Os) are less strong than the M-Cp bonds in ferrocene, ruthenocene, and osmocene, respectively. The η6-C60MC6H6 complexes (M = Cr, Mo, W) should be less stable than their classical analogs (C6H6)M(C6H6). The coordination of a metal atom with the fullerene at its pentagonal face is more energetically favorable than at a hexagonal face.

Mechanisms of the reactions of propane with CBr3+ cation and superelectrophilic complex CBr4+·AlBr3- containing a bromine-centered cationic center: a theoretical study

Fragments of the potential energy surfaces (PES) of the systems [C3H8 + CBr3+] and [C3H8 + Br2CBr+·Br2AlBr2–] were simulated by the MNDO/PM3 method. Energy minima corresponding to weakly bound adducts of propane molecule with the CBr3+ cation or neutral complex CBr3+·AlBr4– were found on the PESs of both systems. These are adducts with the coordination of a H atom of the methylene group of the propane molecule to the electrophile at the Br atom carrying the largest positive charge. As the fragments of the adducts are brought close together, the coordinated H atom migrates to the C atom of the CBr3+ fragment. The potential barriers of these migrations were found to be low for both systems. The reactions proceed without formation of cyclic intermediates or transition states typical of the Olah mechanism.

Crown compounds for anions. The nature of chemical bonds in the complexes of halide anions with cyclic trimeric perfluoro-o-phenylenemercury and some of its analogs

Geometries and electronic structures of the complexes of halide anions with cyclic trimerico-phenylenemercury, (o-C6H4Hg)3, perfluoro-o-phenylenemercury, (o-C6F4Hg)3, vinylenemercury, (C2H2Hg)3, and perfluorovinylenemercury, (C2F2Hg)3, were modelled by the MNDO method. Calculations were performed for [L-X]− semisandwich complexes, [X-L-X]2− bipyramidal complexes, and [L-X-L]− sandwich complexes (where X=Hal,L is a mercury-containing macrocycle). Based on the results of calculations, we concluded that it was advantageous to describe the chemical bonding between halide anions and mercury-containing macrocycles in terms of generalized chemical bonds, which were successfully used for π-complexes of transition metals. In the [L-X]− semisandwich complexes, the halide anion and the metallacycle are involved in the formation of three generalized chemical bonds: one headlight-shaped σ-bond and two two-lobe π-bonds. In the [X-L-X]2− bipyramidal complexes, each halide anion forms three generalized chemical bonds with the macrocycle. It is possible because the macrocycleL has unoccupied molecular orbitals suitable for the formation of such bonds; these MOs consist mainly of the orbitals of mercury atoms directed toward both the upper and lower halogen atoms. In the [L-X-L]− sandwich complexes, the halide anion cannot be bonded to each ringvia three bonds, and, hence, an unsymmetrical structure is formed, in which the rings are located at different distances from the central atom: the [L-X]− semisandwich complex solvated by macrocycleL.

On the mechanism of C ⇄ N migration of alkoxycarbonyl groups in reactions of pyridinium ylides with isocyanates

New examples of reversible C ⇄ N migrations of alkoxycarbonyl groups, which occur in the reactions of pyridinium and 3-(diethylcarbamoyl)pyridinium ylides, viz., derivatives of dimethyl and diethyl malonates, with aryl isocyanates were studied. The mechanism of migration of the methoxycarbonyl group from the carbon atom to the nitrogen atom was considered on the basis of quantum-chemical calculations. The product of the primary attack of the isocyanate group by pyridinium ylide was established to be rearranged with low potential barriers to form carbamate without formation of cyclic intermediate compounds.

MOLECULAR AND ELECTRONIC STRUCTURE OF SOME BOWL-SHAPED UNSATURATED HYDROCARBON MOLECULAR SYSTEMS, POSSIBLE PRECURSORS OF C60 FULLERENE

The AMI calculations of hydrocarbon molecules C20H10 (1) and C30H10 (2) and anions C20H15−4 (3) and C30H15− (4) have been performed with the full energy optimization of the geometric parameters. The chosen topology of the carbon framework of the studied compounds is identical to the topology of the corresponding carbon fragments of C60 fullerene. Analysis of the structure of the frontier orbitals of systems1–4 demonstrated that the tendency of the C20H10 molecule to form sandwich π-complexes of the η5-type with transition metal atoms is small; however, this tendency is significantly larger than that of C30H10. Moreover, the exclusion of the five-membered cycle of bowlshaped unsaturated hydrocarbon molecules and anions from the conjugated system (as a result of the conversion of five framing C atoms to saturated carbon centers) favors the stabilization of their η5-type π-complexes with M(C5H5) species. The stability of these complexes must approach the stability of classical sandwich complexes of the ferrocene type. The results obtained have been extrapolated to polyhedral C60 and C70 carbon clusters.

Rearrangement and cyclization in cross-conjugated reactions of perfluorotriene with tertiary amines

ConclusionsPerfluoro-α,α-bis(isobutenyl)isobutylene reacts with tertiary amines, which are sources of hydride ion. The reaction is accompanied by rearrangement and leads to a cyclic hydrogenation puoduct.

Crystal structure of 2-p-anisidino-2,5-bis (trifluoromethyl)-3-perfluoroisobutenyl-5-monohydroperfluoro-3-hexene

ConclusionsThe structure of 2-p-anisidino-2,5-bis(trifluoromethyl)-3-perfluoroisobutenyl-5-monohydroperfluoro-3-hexene has been determined by x-ray diffraction analysis, proving the occurrence of the 1,4-addition of nucleophiles to a cross-conjugated perfluorotriene.

Crown compounds for anions. MNDO calculations of complexes of halide anions with cyclic pentameric difluoromethylidenemercury

Molecular and electronic structures of complexes of halide anions with cyclic pentameric difluoromethylidenemercury [CF2Hg]5 have been studied by the MNDO method. Calculations have been performed for [L-X]− halfsandwich complexes and bipyramidal complexes [X-L-X]2− having a shape of a spinning top (L is the mercury-containing macrocycle; X = F, Cl, Br, or 1). It has been shown that in complexes of both types, halide anions are bonded to the mercury-containing macrocycle via three generalized chemical bonds: one headlightshaped σ-bond and two two-lobe n-bonds. The complexes studied have been compared with the analogous complexes of the macrocycles containing three mercury atoms. The similarity and differences in the character of the generalized chemical bonds in relation to the size of the cycle have been considered.