Dimeric aggregates are formed by the assembly of two tripodal moieties which are held together, both in solution and in the solid state, by a belt of 6 hydrogen-bonded urea functions.When two or more identical subunits are geometrically and functionally complementary, they may self-assemble to form a super molecule. 1 Urea or thiourea functionalities are excellent candidates to construct molecules that self-assemble by hydrogen-bonding not only in the solid state but are able to persist in solution. 2 Rebek 3 and Böhmer 4 have independently established that calix [4]arenes containing four urea functions at the upper rim form dimers by interdigitation of the all eight ureas in a head-to-tail directional array of 16 hydrogen bonds. Mendoza and coworkers have described dimeric capsules derived from tris(ureido)calix[6]arenes. 5 Herein we report a novel type of self-assembled dimeric aggregate derived from tris(o-ureidobenzyl)amines which resemble the structure of ureidocalixarenes.The tris(ureas) 1-3 6 shown in Scheme 1, easily available 7 from previously reported tris(o-azidobenzyl)amines, 8 showed dramatic differences in their 1 H NMR spectra when recorded in CDCl 3 or DMSO-d 6 . While the spectra of 1-3 in DMSO-d 6 displayed the expected patterns consistent with averaged C 3v symmetries (Fig. 1a), by changing the solvent to CDCl 3 a new set of signals corresponding to a dimer (see below) emerged in each case. The ratio in which both species, monomer and dimer, were present in CDCl 3 depended on the substituent R 2 to a great extent. Thus, while for the tris(urea) 1 (R 2 = p-tolyl) only the signals attributed to the dimer were apparent, for the tris(ureas) 2 and 3 (R 2 = Bn) their spectra were interpreted as corresponding to equilibrium mixtures of both compounds (Fig. 1b).The dimeric species featured two well separated doublets assigned to the diastereotopic methylenic protons of the (ArCH 2 ) 3 N fragment (J gem = 14.5-15.7 Hz), instead of the singlet observed for the same nuclei in DMSO-d 6 . In the dimers emerging from 2 and 3 the ABX systems formed by the three protons of the -CH 2 NH-fragments were clearly evident. The signals assigned to the NH protons of the dimers (near 6.0 and 7.6 ppm, in 2·2 and 3·3) appeared significantly sharp and shifted to lower field when compared to those attributed to the monomers (near 5.2 and 6.3 ppm). The involvement of the urea carbonyl groups as hydrogen bonding acceptors is supported by the d value of their carbon atoms in the 13 C NMR spectra, which were shifted to downfield in CDCl 3 (Dd = 2.8-3.2 ppm). FT-IR of CHCl 3 solutions of 1 (13.7 mM) and 2 (16.4 mM) revealed typical hydrogen-bonded NH-stretching bands 9 at 3317-3327 cm 21 .All those data revealed that the dimers 1·1, 2·2 and 3·3 should possess a highly ordered, hydrogen-bonded structure. Further indication of the existence of 1·1 and 3·3 in solution came from ESI-MS experiments. This technique was used in virtue of its gentle ionization and has been reported to reflect, at least qualitatively, solution phenome...

COMMUNICATIONS 1' as a key intermediate. Preference for electron transfer over hydride transfer in the reaction of 1 with the trityl cation is revealed by competitive atom-transfer trapping of 1 ' with [BrMn(CO),]. These experiments establish a two-step eH mechanism rather than a concerted hydride transfer for the reaction of 1 with the trityl cation. Under the conditions of reduction with H, eH hydride transfer is interrupted, which results instead in the accumulation of Gomberg's dimer derived from one-electron reduction of the trityl cation.Due to the role of 1 in converting H,, a two-electron reducing agent, into a reductant for two one-electron steps, we refer to it as a "redox-switch'' catalyst. Further studies of this unprecedented transfer of electrons from the H -H bond of hydrogen to C-C bonds by one-electron reduction of carbenium ions are in progress.

Several examples of a new type of cage compound, chiral macrobicyclic triphosphazides 15, have been prepared by tripod ± tripod coupling of tris(2-azidobenzyl)amines with 1,1,1-tris[(diphenylphosphino)methyl]ethane (triphos). The structure determination of C 3 or pseudo-C 3 -symmetric compounds 15 revealed their propellerlike topology, which accounted for their chirality, the rare Z configuration of the three phosphazide units, and a new conformation of the triphos fragment. Compounds 15 decomposed in solution with a phosphane arm-off mechanism, to give rise to complex mixtures instead of the expected tri-l 5 -phosphazenes. The stability of 15 in solution was enhanced by the quaternization of the bridgehead nitrogen atom in the form of an N-oxide. Substituents either in the ortho position to the N termini of the phosphazide units or on the benzylic carbon atoms contributed to a decrease in the stability of macrobicycles 15, and in some cases even prevented their preparation.

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