Naturally assembling cocrystallates of C 60 and C 70 fullerenes with tetraphenylporphyrins (H 2 TPP‚C 60 ‚3 toluene, 1; H 2 T 3,5-dibutyl PP‚C 60 , 2; H 2 T 3,5-dimethyl PP‚1.5C 60 ‚2 toluene, 3; H 2 T piv PP‚C 60 , 4; H 2 T 3,5-dimethyl PP‚C 70 ‚4 toluene, 5; ZnTPP‚C 70 , 6; NiT 4-methyl PP‚2C 70 ‚2 toluene, 7) show unusually short porphyrin/fullerene contacts (2.7-3.0 Å) compared with typical π-π interactions (3.0-3.5 Å). In the C 60 structures, an electron-rich, 6:6 ring juncture, C-C bond lies over the center of the porphyrin ring. In the C 70 structures, the ellipsoidal fullerene makes porphyrin contact at its equator rather than its poles; a carbon atom from three fused six-membered rings lies closest to the center of the porphyrin. These structures provide an explanation for the manner in which tetraphenylporphyrin-appended silica stationary phases effect the chromatographic separation of fullerenes. The interaction of the curved π surface of a fullerene with the planar π surface of a porphyrin, without the need for matching convex with concave surfaces, represents a new recognition element in supramolecular chemistry. NMR measurements show that this interaction persists in toluene solution, suggesting a simple way to assemble van der Waals complexes of donor-acceptor chromophores.

No example of a simple uncatalyzed dimerization of a diaminocarbene has been clearly established, so it is timely to ask what factors control the thermodynamics of this reaction, and what mechanisms are responsible for the observed dimerizations? In agreement with qualitative experimental observations, the dimerizations of simple five- and six-membered-ring diaminocarbenes are calculated to be 100 kJ mol(-1) less favorable than those of acyclic counterparts. This large difference is semiquantitatively accounted for by bond and torsional angle changes around the carbene centers. Carbenes such as (Et(2)N)(2)C are kinetically stable in THF at 25 degrees C in agreement with calculated energy barriers, but they rapidly dimerize in the presence of the corresponding formamidinium ion. This proton-catalyzed process is probably the most common mechanism for dimer formation, and involves formation of C-protonated dimers, which can be observed in suitable cases. The possibility of alkali-metal-promoted dimerization is raised, and circumstantial evidence for this is presented.

Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular recognition element is explored in discrete, soluble, coordinatively linked porphyrin and metalloporphyrin dimers. Jawlike clefts in these bis-porphyrins are effective hosts for fullerene guests. X-ray structures of the Cu complex with C60 and free-base complexes with C70 and a pyrrolidine-derivatized C60 have been obtained. The electron-rich 6:6 ring-juncture bonds of C60 show unusually close approach to the porphyrin or metalloporphyrin plane. Binding constants in toluene solution increase in the order Fe(II) < Pd(II) < Zn(II) < Mn(II) < Co(II) < Cu(II) < 2H and span the range 490−5200 M-1. Unexpectedly, the free-base porphyrin binds C60 more strongly than the metalated porphyrins. This is ascribed to electrostatic forces, enhancing the largely van der Waals forces of the π−π interaction. The ordering with metals is ascribed to a subtle interplay of solvation and weak interaction forces. Conflicting opinions on the relative importance of van der Waals forces, charge transfer, electrostatic attraction, and coordinate bonding are addressed. The supramolecular design principles arising from these studies have potential applications in the preparation of photophysical devices, molecular magnets, molecular conductors, and porous metal-organic frameworks.