NMR studies of the tautomerism of cyclo-tris(4-R-2,6-pyridylformamidine) in solution and in the solid state

Two novel cyclo-tris(4-R-2,6-pyridylformamidine)s (R = H, CH3) have been studied by solution-and solid-state NMR. Both compounds show fast exchange of NH protons in dimethyl sulfoxide at room temperature. The E-syn configuration of the formamidine group with the NH protons in the outer ring position could be proved by low-temperature H-1 NMR experiments. The influence of deuteration of the NH group on the exchange rate has been demonstrated qualitatively. Proton exchange at the formamidine groups results in both a symmetric (S) and an asymmetric (A) isomer which could be spectroscopically identified and characterized at 193 K. Whereas two degenerate forms exist for S, six degenerate A forms can be distinguished. Prototropic tautomerism at one formamidine group results in exchange from S into A, whereas A is transformed to a degenerate A form or to S. It could be shown that some transitions between substructures are impossible by a single -N*H-CH=N-/-N*=CH-NH- exchange. The S isomer with three equivalent formamidine groups is the preferred isomer in solution as indicated by the S/A ratio at 193 K. From this result we conclude that in polyformamidines, ordered sequences of formamidine tautomers are also formed at low temperatures. Prototropic exchange was not observed in the solid state, neither by C-13 nor by N-15 solid-state NMR, For one trimer (R = CH3), three molecules dimethyl sulfoxide per trimer molecule are within the lattice as could be proved by C-13 CPMAS NMR.