The Small Cage 12,17-dimethyl-5-oxa-1,9,12,17-tetra-azabicyclo[7.5.5]nonadecane (l) - Its Synthesis, Characterization, and Proton Sponge Behavior - the Crystal-structure of the Dipicrate Salt [h2(l)](picrate)2

The synthesis and characterization of the new oxa-azamacrobicycloalkane 12,17-dimethyl-5-oxa-1,9,12,17-tetra-azabicyclo[7.5.5]nonadecane () are described. Its basicity behaviour in both aqueous and water-MeSO (50:50 mol/mol) solutions has been investigated by potentiometric and spectroscopic (H and C n.m.r.) techniques. In aqueous solution it behaves as a very strong base (proton sponge) in the first protonation step, and as a relatively strong base in the second step (log = 11.21). In the mixed water-MeSO solvent both protonation constants have been measured: log = 14.0 and log = 8.2. H N.m.r. experiments indicate that in the monoprotonated species H() the proton is rapidly exchanged with acidic hydrogens on the n.m.r. timescale. H-H and H-C two-dimensional n.m.r. experiments allowed the unequivocal assignment for all the H proton and C resonances of both species H() and H(). Crystals of [H()](picrate) are monoclinic, space group 2/, with = 12.868(8), = 17.201(2), = 16.614(2)A, [small beta]= 110.18(1)[degree], and = 4; final values of 0.090 (= 0.090) for 3 801 observed reflections. The four basal nitrogen atoms and the apical oxygen atom are located at the apices of a slightly distorted square pyramid. -Ray analysis shows that protonation occurs on the NCH, groups. Each hydrogen atom of the NHCH interacts with the oxygen atom, the 0 H distances 2.328(16) and 2.112(18)A, and with both bridgehead nitrogen atoms, the H N distances ranging from 2.32 to 2.34 A. This arrangement makes the overall structure very stable from the thermodynamic point of view and explains the unusually high basicity of () in both first- and second-protonation step