Surprising acid/base and ion-sequestration chemistry of Sn9(4-): HSn9(3-), Ni@HSn9(3-), and the Sn9(3-) ion revisited

K(4)Sn(9) dissolves in ethylenediamine (en) to give equilibrium mixtures of the diamagnetic HSn(9)(3-) ion along with K(x)Sn(9)((4-x)-) ion pairs, where x = 0, 1, 2, 3. The HSn(9)(3-) cluster is formed from the deprotonation of the en solvent and is the conjugate acid of Sn(9)(4-). DFT studies show that the structure is quite similar to the known isoelectronic RSn(9)(3-) ions (e.g., R = i-Pr). The hydrogen atom of HSn(9)(3-) (δ = 6.18 ppm) rapidly migrates among all nine Sn atoms in an intramolecular fashion; the Sn(9) core is also highly dynamic on the NMR time scale. The HSn(9)(3-) cluster reacts with Ni(cod)(2) to give the Ni@HSn(9)(3-) ion containing a hydridic hydrogen (δ = -28.3 ppm) that also scrambles across the Sn(9) cluster. The Sn(9)(4-) ion competes effectively with 2,2,2-crypt for binding K(+) in en solutions, and the pK(a) of HSn(9)(3-) is similar to that of en (i.e., Sn(9)(4-) is a very strong Br?nsted base with a pK(b) comparable to that of the NH(2)CH(2)CH(2)NH(-) anion). Competition studies show that the HSn(9)(3-) ? Sn(9)(4-) + H(+) equilibrium is fully reversible. The HSn(9)(3-) anion is present in significant concentrations in en solutions containing 2,2,2-crypt, yet it has gone undetected for over 30 years.     

Stereospecific [2 + 2]-cross-photocycloaddition in a supramolecular donor–acceptor complex

A bis(propylammonium) derivative of (E)-4-(4-mercaptostyryl)pyridine, which was synthesized for the first time, forms a highly stable bimolecular complex with a bis(18-crown-6 ether) derivative of (E)-stilbene in solution owing to ditopic coordination via hydrogen bonds. The complex formation results in much faster deactivation of the excited states of both compounds, which is explained by photoinduced electron transfer from the stilbene derivative to the styrylpyridinium dye. Despite this, the complexed olefins undergo [2?+?2]-cross-photocycloaddition upon selective excitation of the dye to afford solely the syn-cycloadduct. The retro-photocycloaddition occurs readily upon UV irradiation of the cycloadduct and leads to the initial bimolecular complex.     

Two-dimensional electrons in spirally rolled-up quantum wells

An effective Hamiltonian is obtained to describe the motion of a one-dimensional quantum particle along an arbitrary plane curve. Calculations are made of the energy levels and the polarization dependence of the electromagnetic wave absorption in a spirally rolled-up quantum well.     

Magnetic and crystalline properties of GdxLa1−x FeSi compounds

The magnetic and crystalline properties of the polycrystalline compounds Gd_xLa_1−x FeSi (x=0, 0.1, 0.3, 0.45, 0.55, 0.65, 0.8, 1) and single crystal GdFeSi are studied. All the compounds have the CeFeSi tetragonal structure. The temperature dependences of the magnetization and initial susceptibility show that ferromagnetic behavior is characteristic of these compounds; the exception is the Pauli paramagnetic LaFeSi. Substitution of gadolinium by the nonmagnetic lanthanum leads to a sharp drop in the Curie temperature T _c, and to a reduction in the saturation magnetic moment μ ₀, effective magnetic moment μ _eff, and paramagnetic Curie temperature Θ_p. Measurements on single crystal GdFeSi imply that the easy magnetization axis of this compound is the c axis and the anisotropy field is H _a∼4 kOe. No magnetic moment was observed on the iron ion in any of these compounds. Fiz. Tverd. Tela (St. Petersburg) 39, 325–329 (February 1997)     

Synthesis of formyl derivatives of benzocrown ethers containing N,S, and O heteroatoms in the macrocycle

A method for the synthesis of 4-bromobenzodithia-15(18)-crown-5(6) and 4-bromobenzodiaza-15 (18)-crown-5(6) by condensation of 3,4-di(2-haloethoxy)bromobenzene with polyoxaalkanes containing terminal SH or NHMe groups was suggested. The method for the synthesis of formyl derivatives of benzocrowns containing N, S, and O heteroatoms in the macrocycle based on the metallation of appropriate bromo derivatives with BuⁿLi followed by treatment of the resulting organolithium intermediates with DMF was developed. Oximes and semicarbazones of benzaldehydes containing a crown ether fragment were obtained, and their transformation into the original aldehydes by treatment with KNO₂ in an acid medium was studied.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 121–128, January, 1995.This study was financially supported by the Russian Foundation for Basic Research (Grant No. 94-03-08-531) and the International Science Foundation (Grant M8Q000).     

Styryl dyes. Synthesis and study of the solid-state [2+2] autophotocycloaddition by NMR spectroscopy and X-ray diffraction

New styryl dyes of the pyridine and benzothiazole series were synthesized with the aim of investigating the solid-state [2+2] autophotocycloaddition (PCA) reaction. The ¹H NMR spectroscopy showed that for most of the compounds under study, the visible light irradiation of thin polycrystalline films of the dyes affords cyclobutane derivatives. The rate of the photoreaction depends on the structure of the dye and is higher for compounds, which contain a short N-substituent in the heterocyclic moiety and have strong absorption in the visible region. Dyes bearing electron-releasing substituents in the benzene ring undergo the stereospecific PCA in the syn-head-to-tail dimeric pair to give the only rctt isomer of cyclobutane derivatives. Electron-withdrawing and bulky substituents in the benzene fragment of styryl dyes extend the range of the mutual orientations of the molecules in the dimeric pairs, resulting in the formation of two or even four isomeric cyclobutanes in the PCA reactions. The structures of some dyes were established by X-ray diffraction. In the overwhelming majority of the structures, one of two packing modes, either syn-head-to-tail or syn-head-to-head, with extensive stacking interactions is observed. A rare example of the anti-head-to-head stacking mode was found for the dicationic dye containing the bulky N⁺(Et)Me₂ substituent in the benzene ring. The syn-head-to-tail and anti-head-to-head stacking modes can facilitate the PCA reaction due to the close spatial proximity of the ethylenic bonds and their parallel orientation in the dimeric pairs of the dye molecules. Dedicated to Academician G. A. Abakumov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1797–1819, September, 2007.     

Stereospecific solid-state [2+2] autophotocycloaddition of a styryl dye containing a 18-crown-6 fragment

A procedure was developed for the synthesis of a 18-crown-6-containing styryl dye of the pyridine series. Phototransformation products of the dye that formed upon irradiation with visible light in solution, films, and crystals were studied by ¹H and ¹³C NMR spectroscopy and spectrophotometry. Irradiation of solutions of this dye in acetonitrile leads only to reversible E-Z photoisomerization of the C=C bond. Irradiation of a film of the dye induces stereospecific [2+2] photocycloaddition to form exclusively the rctt isomer of tetrasubstituted cyclobutane. The latter was found to undergo base-catalyzed isomerization giving rise to the rcct isomer. X-ray diffraction study showed that photocycloaddition occurs in single crystals without their destruction. The structures of the E isomer of the dye and the resulting rctt isomer of the cyclobutane derivative were established. The characteristic features of the molecular packing of the dye favorable for topochemical photocycloaddition in the crystal and the structural changes that accompany the cyclobutane formation are discussed. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1896–1908, August, 2005.     

Building up of Macroring in the New Synthesis of Azacrown Ethers. Structure and Complex Formation of Nitrobenzoazacrown Ethers

A cyclization of azapodand haloderivatives into nitrobenzoazacrown ethers under treatment with various bases and in their absence was investigated. The nitrobenzoazacrown ethers obtained and their complexes with metal cations were studied by X-ray diffraction method and by ¹H NMR titration. In nitrobenzoaza-15-crown-5 a capability to complex Ca²⁺ cation was found that significantly exceeded similar ability of nitrobenzocrown ether with the same size of the macroring.     

An electron-excessive nitrosyl complex: reactivity of a ligand-centered radical leading to coordinated HNO

The reaction of RuHCl(CO)L(2) (L = P(i)Pr(3)) with NO initially forms a 1:1 adduct, shown by DFT calculations and EPR spectroscopy (including the RuD isotopomer) to contain a bent ( 90 degree angle Ru-N-O = 143.9 degrees ) nitrosyl where the majority of the spin density is on the nitrosyl nitrogen. This radical adduct transforms further to give equimolar RuCl(NO)(CO)L(2) and RuHCl(HNO)(CO)L(2), the latter with hydride trans to the nitroxyl ligand HN=O. This is the first observation of the synthesis of coordinated HNO from NO itself. DFT calculations lead to the proposal that this H-atom transfer is effected by free NO, and the lifetime of RuHCl(HNO)(CO)L(2) is indeed qualitatively dependent on the presence of free NO.     

Competitive aryl-iodide vs aryl-aryl reductive elimination reactions in Pt(IV) complexes: experimental and theoretical studies

The platinum(IV) complex trans-(dmpe)Pt(IV)(Ar)2I2 (2, dmpe = 1,2-dimethylphosphinoethane, Ar = 4-FC6H4) rapidly reacts, upon moderate heating in solution under ambient light, via two distinct pathways: isomerization to the corresponding cis-isomer (3) and Ar-I reductive elimination to give (dmpe)Pt(II)(Ar)I (4). Complex 3 undergoes, upon prolonged heating at high temperatures, an exclusive Ar-Ar reductive elimination reaction to give (dmpe)Pt(II)I2. Experimental and DFT studies showed that the 2-to-3 isomerization proceeds via three pathways: photochemical or thermal phosphine chelate opening and a mechanism involving cleavage of the Pt-I bond. The isomerization reaction is significantly slowed down but not stopped in the absence of light or in the presence of an excess of tetra-n-butylammonium iodide. On the other hand, the Ar-I reductive elimination from 2 proceeds via the Pt(delta+)-I(delta-) ion pairlike transition state. Use of the rigid dmpe analogue 1,2-dimethylphosphinobenzene (dmpbz) as the ligand shuts down the chelate ring-opening isomerization pathway and enables faster Ar-I reductive elimination thus making the latter reaction the major reaction route for the dmpbz supported trans-diiodo Pt(IV) complex 8.