Chemical Exchange in Novel SpiroBicyclic Zwitterionic Janovsky Complexes using Dynamic 1H NMR Spectroscopy

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Journal titleMagnetic Resonance in Chemistry
Pages158164; # of pages: 7
SubjectNMR; 1H; 13C; EXSY; saturation transfer; proton exchange; Janovsky σ-complex; spirohydantoin; NMR modelling
AbstractHighly 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) 1H NMR techniques. The coalescence temperature was determined to be 62 ◦C ind3-acetonitrile. Analysis of these data provided a Gibbs free energy of activation, ∆G‡, of +67 kJ mole−1, a rate constant, k, coalescence of 220 Hz and an equilibrium constant, Keqm, of 0.98 as estimates of the exchange process in this solvent. Of the two mechanisms proposedfor this fluxional behaviour, ringopeningto a substituted benzene or proton exchange, a further theoretical modelling study of 1D 1H 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 d3-acetonitrile (to 75 ◦C) and d6-dimethyl sulfoxide (DMSO). Molecular modelling at the MM2 level suggests that steric collisions of anN-acyl isopropyl substituent of the hydantoin ringwith the ortho-nitro group of the spirofused cyclohexadienyl ring prevents the proposed proton exchange mechanism occurring in this case.
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AffiliationNRC Institute for Information Technology; National Research Council Canada
Peer reviewedYes
NRC number50718
NPARC number16907876
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Record identifier44d7a44a-c9a9-4005-a12c-7f0107e63831
Record created2011-02-22
Record modified2016-05-09
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