Details

Autor(en) / Beteiligte

• Culf, A. S.
• Čuperlović‐Culf, M.
• Ouellette, R. J.

Titel

Chemical exchange in novel spirobicyclic zwitterionic Janovsky complexes using dynamic 1 H NMR spectroscopy

Ist Teil von

• Magnetic resonance in chemistry, 2009-02, Vol.47 (2), p.158-164

Erscheinungsjahr

2009

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Abstract Highly coloured Janovsky complexes have been known for over 120 years, being used in many colourimetric analytical procedures. In this present study, two novel and stable nitrocyclohexadienyl spirobicyclic, zwitterionic Janovsky anionic hydantoin σ‐complexes, rac‐1,3‐diisopropyl‐6‐nitro‐2,4‐dioxo‐1,3‐diazaspiro[4.5]deca‐6,9‐dien‐8‐ylideneazinate, ammonium internal salt (1) and 1,3‐diisopropyl‐2,4‐dioxo‐1,3‐diazaspiro[4.5]deca‐6,9‐dien‐8‐ylideneazinate, ammonium internal salt (2) have been prepared and characterised by NMR, electrospray ionization mass spectrometry (ESI‐MS) and UV/visible methods. For the p ‐mononitro‐substituted complex (2), we discovered chemical exchange behaviour using 1D saturation transfer and 2D exchange spectroscopy (EXSY) 1 H NMR techniques. The coalescence temperature was determined to be 62 °C in d 3 ‐acetonitrile. Analysis of these data provided a Gibbs free energy of activation, Δ G ‡, of + 67 kJ mole −1 , a rate constant, k , coalescence of 220 Hz and an equilibrium constant, K eqm , of 0.98 as estimates of the exchange process in this solvent. Of the two mechanisms proposed for this fluxional behaviour, ring opening to a substituted benzene or proton exchange, a further theoretical modelling study of 1D 1 H NMR spectra was able to confirm that simple proton exchange between the two nitrogen sites of the hydantoin ring provided an accurate simulation of the observed experimental evidence. Interestingly, the o , p ‐dinitro‐substituted complex (1) did not show any chemical exchange behaviour up to 150 °C in d 3 ‐acetonitrile (to 75 °C) and d 6 ‐dimethyl sulfoxide (DMSO). Molecular modelling at the MM2 level suggests that steric collisions of an N ‐acyl isopropyl substituent of the hydantoin ring with the ortho ‐nitro group of the spirofused cyclohexadienyl ring prevents the proposed proton exchange mechanism occurring in this case. Copyright © 2008 Crown in the right of Canada. Published by John Wiley & Sons, Ltd