engleski

Dimerization of aromatic C-nitroso compounds: insights from NMR spectroscopy

Aromatic C-nitroso compounds can dimerize by forming Z- or E-azodioxides. In the solid state, aromatic C-nitroso compounds are usually present as dimers, and in solution monomer-dimer equilibrium is established. At room temperature the equilibrium is usually shifted towards the nitroso monomers, while Z- and/or E-azodioxides can be observed only at low temperatures. Our studies revealed that aromatic C-nitroso compounds are also able to form asymmetrical dimers or heterodimers. The selectivity in formation of heterodimers was examined by investigating cross- dimerization of several p- and m-substituted nitrosobenzene derivatives with the parent nitrosobenzene in solution by one- and two- dimensional variable temperature 1H NMR, and in solid state by 13C CP-MAS NMR and IR spectroscopy. It was found that in solution and in solid state the selectivity is different. The selectivity is in both media affected by the electron-donating ability of the substituent, and the differences can be explained by the influence of molecular arrangements in the crystal lattice. Recently, we investigated solution-state monomer-azodioxide equilibria and conformational freedom of several new aromatic dinitroso derivatives by using one- and two-dimensional variable temperature 1H NMR spectroscopy and quantum chemical calculations. Compounds with multiple aromatic nitroso groups are interesting because they can form oligomeric and polymeric structures through azodioxide bonds, which makes them promising candidates for design of new supramolecular systems. Inspection of NMR spectra revealed that the studied compounds dimerize by lowering the solution temperature yielding the mixture of monomers and Z- and/or E- azodioxides. Experimental NMR results were validated by DFT calculations enabling assignments of proton chemical shifts of azodioxy species. The DFT was used to predict the geometric (Z/E) preference of the azodioxy dimers revealing higher stability of the Z-form.

Aromatic C-nitroso compunds ; Dimerization ; NMR spectroscopy

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