Carbanion substituent effects on 1-disubstituted 4-(4'-pyridyl)pyridinium methylide structures using 13C NMR spectroscopy and DFT method.

The structure of 1-disubstituted 4-(4'-pyridyl)pyridinium methylides or 4,4'-bipyridinium monoylides (2-5) with a wide range of carbanion substituents, were determined using 13C NMR signals in dimethylsufoxide (DMSO-d(6)) solution. For the first time, we developed a systematic determination of 13C NMR chemical shifts of the ylidic carbon using a long-range correlated (1H-(13)C) HMBC experiments. The chemical shift values are discussed in terms of magnetic and/or electronic effects of the ylidic carbon substituents. From the extracted NMR parameters and the results of accompanying quantum chemical DFT calculations for a three-dimensional (3D)-structure representation, we found a long distance electronic effect where the aromatic heterocycle C2z.sbnd;C6 and C4 centers are perturbed according to the electron acceptor strengths of ylidic carbon substituents in all monoylides (2-5c) capable to stabilize in a planar conformation. No significant perturbation on C2z.sbnd;C6 and C4 centers are found in all other monoylides (2-5a, b) that adopted a non-planar conformation. Good similar linear dependences of the chemical shift variation Delta (calculated by the differences of analogous C2z.sbnd;C6 and C4 chemical shifts in non-planar and planar monoylides) with the ylidic carbon chemical shifts modulated by the strength of electron acceptor substituents pointed out the resonance interaction or the delocalization phenomena of the ylidic carbon charge on the heterocycle.