No abstract

New cyclic metalloporphyrin hosts, 6 and 7, have been prepared. At 0.33 mM in dichloromethane at 25°C, they accelerate 65-fold and 840-fold respectively the reaction of diene 1 and dienophile 2 and also bind the hetero Diels-Alder product 3 very strongly. More importantly, small single crystals of solvated 6, 7, and the 6 . 3 complex were grown and their structures were determined. As the Diels-Alder product resembles the Diels-Alder transition state, the structures of the product-free host 6 and the 6‚3 host-product complex allow, for the first time for synthetic receptors, a detailed structural analysis of the geometrical changes imposed on an accelerating agent on binding of a Diels-Alder product. Comparison of these structures reveals that when the Diels-Alder product 3 is bound within the cavity, it induces significant structural changes in 6. This provides the first crystallographic structural evidence that accelerated product formation can be accompanied by substantial host distortion. Desolvation of host and guests emerges as another factor, implying that solvent stabilization is not as significant for the host-accelerated reaction as in the control (host free) reaction.

The coordination chemistry of a Rh(III) porphyrin building block was investigated with a view to the construction of heterometallic arrays of porphyrins. The Rh(III) porphyrin was found to coordinate methanol in the solid state and weakly in CDCl(3) solution. Crystallization afforded five coordinate pi stacked Rh(III) porphyrins. The distribution of products from reaction of Rh(III) porphyrin with DABCO, 4,4'-bipyridine, and 4,4'-bipyrimidine could be displaced toward dimeric species by silica gel column chromatography or recrystallization which served to remove excess ligand. Weak coordination to nitriles was observed, although it was sufficiently strong to organize a dimeric complex of 5,5'-dicyano-2,2'-bipyridine in the solid state. Complexes with 4,4'-bipyrimidine and 5,5'-dicyano-2,2'-bipyridine possess uncoordinated chelating nitrogen atoms. Larger heterometallic porphyrin arrays were assembled using a combination of Sn(IV) and Rh(III) porphyrin coordination chemistry. A Sn(IV) porphyrin acted as a core around which were coordinated two isonicotinate groups, carboxylic acid functionalized porphyrins, or porphyrin trimer dendrons. Rh(III) porphyrins were coordinated to pyridyl groups at the periphery of these entities. In this way an eleven porphyrin array, with four different porphyrin metalation states, was assembled. The diamagnetic nature of both the Rh(III) and Sn(IV) porphyrins, the slow ligand exchange kinetics on the NMR time scale, and tight ligand binding permitted the porphyrin arrays to be analyzed by two-dimensional (1)H NMR techniques.