COMBINED X-RAY-DIFFRACTION AND N-15 CPMAS NMR-STUDY OF MOLECULAR-STRUCTURE AND PROTON ORDER DISORDER PHENOMENA IN CYCLIC N,N'-BISARYLFORMAMIDINE DIMERS/
R. Anulewicz et al., COMBINED X-RAY-DIFFRACTION AND N-15 CPMAS NMR-STUDY OF MOLECULAR-STRUCTURE AND PROTON ORDER DISORDER PHENOMENA IN CYCLIC N,N'-BISARYLFORMAMIDINE DIMERS/, Journal of the American Chemical Society, 119(50), 1997, pp. 12223-12230
Crystal structures of a series of five symmetrically substituted N,N-b
isarylformamidines ArNH-CH=NAr with Ar = X-C6H4, X = p-OCH3 (IV), p-CH
3 (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, th
e 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 diff
erent tautomers, a and b, interconverting by fast double proton transf
ers. The NMR experiments indicate three types of amidines characterize
d by different magnitudes of the equilibrium constants K-ab of the tau
tomerism. In dimers of type such as V-VIII, we find K-ab much less tha
n 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 to
rsional and valence angles involving the two aryl groups of an amidine
unit are different. For dimers such as II and III, characteristic tem
perature dependent N-15 CPMAS NMR line shape changes are observed indi
cating 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. Howev
er, 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 lab
eled as ''dynamic partial order''. The implications of the molecular s
tructure and the hydrogen bond and proton transfer characteristics are
discussed.