Structures 1 and 2 are self-complementary but they are also mutually well suited. The hybrid aggregate (1 -2) of 1 and 2 is shown in Scheme 3. The calculated structure of the pear shaped 1 -2 adducts suggests the formation of only six "strong" hydrogen bonds. The two additional bonds are significantly longer.
Scheme 3 TWCI view\ ofMM2-minimized hybrid dimer 1 -2. Some hydrogen atoms and the ester groups have been omitted for clarity.
In the experiment 1 -I and 2-2 were shown to be capable of recombination to form hybrid species. Specifically, the ' H N M R spectrum of an equimolar mixture of 1 and 2 in CDC1, showed signals corresponding to three different species. Besides the signals of the two homodimers 1-1 and 2-2, a third set of signals, which we assigned to the "heterodimer" 1-2, was present. This assignment was supported by NOE experiments: at -40 'C a strong NOE was observed in the heterodimer's spectrum on irradiation of the NH protons of either of its half shells, I and 2. The disproportionation constants K and the thermodynamic values for the hybrid formation were determined by ' H N M R spectroscopy. In CDCI, K is 11.6 at 298 K (titrations of 1 with 2 and vice versa gave the same value). The enthalpy for heterodimerization in CDCI, was measured to be -3 kcalmol-' and the entropy -5 calK-'mol-'.
What is the driving force for the recombination, particularly if hydrogen bonding is better in 1 -1 than in 1 -2? Nucleation by guest species is the likely answer. The distribution of the hydrogen-bonded pairs at equilibrium is influenced by the presence of suitable guests. Though all three pairs are capable of accommodating guest molecules, the cavity size of each varies. For instance, the dimer 1-1 readily encapsulates CH,CI, but not CHCI,. Only the bridged anthrdcene dimer 2-2 appears large enough to encapsulate CHCI,CHCI,, but both 2-2 and the heterodimer 1 ~ 2 are large enough to accommodate CHCI, or even CHBr, These surmises are borne out by the data of Table 1 : in CHCI, (or even more so in CHBr,) the formation of the hybrid ball 1-2 Table I . tqtiilihriiiin conarants for the disproportionation (recombination) of I -1 ;ind 2 2 i n soIvtiit\ of increasing ske. sol\enr CD,CI, CDCI, CDBr, CDCI,CDCI, 11 6
no is favored, but very little heterodimer 1-2 is formed in CD,Cl, or CDCI2CI)CI,. These results suggest that a species is favored if its cavity can be well-filled.
Further evidence for these nucleation effects was provided by titration experiments and will be reported elsewhere. In the meantime, the surprisingly stable heterodimer found here augurs well for the formation of new hybrids from other self-assembly processes.