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Solution and Solid State NMR Studies of Cross-Dimerization of Nitrosobenzenes

Nitrosobenzenes can exist as blue or green monomers or as colorless azodioxy dimers in two forms, (Z) and (E). In the solid state most of them exist as dimers, while in solution monomer-dimer equilibrium is established. Previous temperature dependent NMR studies have shown that while at ambient temperature the main solution species are monomers decreasing temperature favours the dimeric azodioxy species with both (Z)- and (E)-forms present in addition to the monomer.[1] In the present work we studied cross-linking of nitrosobenzenes to asymmetricaly substituted azodioxides (heterodimers) in cases when they are not sterically crowded with large groups in ortho-position what was previously stated as the necessary condition for heterodimer formation.[2] The formation of heterodimers was studied in the series of para- and meta-substituted nitrosobenzenes both in solution (by one- and two-dimensional 1H NMR spectroscopy) and in solid state (by CP-MAS 13C NMR spectroscopy). Equimolar amounts of two different nitrosobenzenes were dissolved in CDCl3, thus obtained solution was cooled to 218 K and 1H NMR COSY spectra were recorded and analyzed. Heterodimer formation was prooved if obtained NMR spectra were different from the sum of spectra recorded for the solutions of individual components under the same conditions. In the case of cross-dimerization study in solid state, we compared 13C CP-MAS spectra of solids obtained after mixing the equimolar solutions of two different nitrosobenzenes and evaporation of the solvent and sum of spectra of pure homodimers. Differences in these two spectra are indication of heterodimer formation. We have shown that nitrosobenzenes can form heterodimers in solution as well as in solid state even when they are not sterically crowded with large groups in ortho-position.[3] The ability of heterodimer formation is quite different in solid state from that in solution probably because of a considerable influence of packing factors present in the former. [1] Fletcher, D. A. ; Gowenlock, B. G. ; Orrell, K. G. J. Chem. Soc., Perkin Trans 2 1997, 2201. [2] Batyuk, V. A. ; Shabatina, T. I. ; Morozov, Yu N. ; Ryapisov, S. V. ; Sergeev, G. B. Vestn. Moscow. Univ. Seriya 2. Chimia. 1988, 29(No5), 270. [3] Halasz, I. ; Biljan, I. ; Novak, P. ; Meštrović, E., Plavec, J. ; Mali, G. ; Smrečki, V. ; Vančik, H. Journal of Molecular Structure 2008, in press.

Cross-Dimerization; Nitrosobenzenes; NMR spectroscopy

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