Combined X-ray diffraction and 15N CPMAS NMR study of molecular structure and proton order/disorder phenomena in cyclic N,N′-bisarylformamidine dimers

Crystal structures of a series of five symmetrically substituted N,N-bisarylformamidines ArNH-CH=NAr with Ar = X-C6H4, X = p-OCH3 (IV), p-CH3 (V), p-F (VI), p-NO2 (VII), and m-Br (VIII) have been determined by single-crystal X-ray diffraction (XRD) and complete the series studied previously where X = H(I), p-Br (II), and p-Cl (III). In addition, the results of variable-temperature N-15 CPMAS NMR experiments performed on N-15-labeled I, II, and IV are reported. All compounds form cyclic dimers linked by two N-H ... N hydrogen bonds which can form two different tautomers, a and b, interconverting by fast double proton transfers. The NMR experiments indicate three types of amidines characterized by different magnitudes of the equilibrium constants K-ab of the tautomerism. In dimers of type such as V-VIII, we find K-ab much less than 1 (i.e., only a single tautomer in the temperature range between 100 and 300 K). In this case, the hydrogen-bonded protons are ordered and can be localized by XRD. Furthermore, the C ... N bond lengths and torsional and valence angles involving the two aryl groups of an amidine unit are different. For dimers such as II and III, characteristic temperature dependent N-15 CPMAS NMR line shape changes are observed indicating that K-ab = 1 within the margin of error. Rate constants of the tautomerism can in this case be obtained by line shape analysis. For this degeneracy to occur, the aryl group conformations at both amidine nitrogen atoms must be similar. XRD then observes disordered hydrogen-bond protons and, in principle, also disordered nitrogen atoms. However, in practice, the disorder of the latter is not resolved leading to the observation of equalized C ... N bond lengths. Finally, dimers (I, IV) represent an intermediate case with K-ab < 1, which could be labeled as "dynamic partial order". The implications of the molecular structure and the hydrogen bond and proton transfer characteristics are discussed.