Using dynamic NMR spectroscopy, the kinetics of the degenerate double proton transfer in cyclic dimers of polycrystalline (15)N,(15)N'-di-(4-bromophenyl)-formamidine (DBrFA) have been studied including the kinetic HH/HD/DD isotope effects in a wide temperature range. This transfer is controlled by intermolecular interactions, which in turn are controlled by the molecular conformation and hence the molecular structure. At low temperatures, rate constants were determined by line shape analysis of (15)N NMR spectra obtained using cross-polarization (CP) and magic angle spinning (MAS). At higher temperatures, in the microsecond time scale, rate constants and kinetic isotope effects were obtained by a combination of longitudinal (15)N and (2)H relaxation measurements. (15)N CPMAS line shape analysis was also employed to study the non-degenerate double proton transfer of polycrystalline (15)N,(15)N'-diphenyl-formamidine (DPFA). The kinetic results are in excellent agreement with the kinetics of DPFA and (15)N,(15)N'-di-(4-fluorophenyl)-formamidine (DFFA) studied previously for solutions in tetrahydrofuran. Two large HH/HD and HD/DD isotope effects are observed in the whole temperature range which indicates a concerted double proton transfer mechanism in the domain of the reaction energy surface. The Arrhenius curves are non-linear indicating a tunneling mechanism. Arrhenius curve simulations were performed using the Bell-Limbach tunneling model. The role of the phenyl group conformation and hydrogen bond compression on the barrier of the proton transfer is discussed.

The electrophilic character of tin in the new compounds 3 is manifested in reactions with n~c l e o p h i l e s .~'~~ With pyridine and with 2,2'-bipyridyl, 3a forms the adducts 6a and 7a, ["] in which the tin atom is coordinatively saturated (cf. Table 1, ' 9Sn resonance signals at high field). As in the case of 3a, dynamic processes can also be demonstrated for 6a and 7a by 'Hand I3C-NMR spectroscopy. The structure for the intermediates 4 is based on consistent sets of NMR data (cf. Table 1 for 4b) and is confirmed by the isolation of the final product 5. The results suggest that the formation of compounds 4 and 5 from 3 and 4 respectively proceeds via an Sn-Substituted vinyl cation 8. [Z 4586 IE] German version : Angew. Chem. 103 (1 991) 1374 CAS Reeistrv numbers:[9] 3b, 3c, 3d, 3e: relevant 'Hand I3C NMR data correspond to the values for 3a. 119Sn/11B NMR (CD,CI,, 298 K): 6("9Sn)/6(1'B) = + 207.5/ -4.9 (3b); + 203.1/-4.5 (3c); + 2232-4.3 (3d); + 202.1/-4.4 (3e).[lo] R. Boese, D. Blaser, J Appl. Crysrallogr. 22 (1989) 394.[11] X-ray structure analysis of 3a at 115 K : P2,/c, a = 11.924 (2), b = 14.512(3), c = 14.299(2) A, a = y = 90". = 97.29(1)", V = 2454.4(7) A3, 2 = 4, e,.,,, = 1.274 gcm3, p = 1.05 mm-', Mo,. radiation (graphite monochromator), i . = 0.71069 A; 3414 independent reflections. 2979 observed (4 2 4c(F)); 2@(max) = 45"; absorption correction: empirical, Y scans in the 28 range, r): 3 2 35 max/min. Transmission: 0.926/0.815, R,,,,

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