Abstract
Vernier templating exploits a mismatch between the number of binding sites in a template and a reactant to direct the formation of a product that is large enough to bind several template units. Here, we present a detailed study of the Vernier-templated synthesis of a 12-porphyrin nanoring. NMR and small-angle X-ray scattering (SAXS) analyses show that Vernier complexes are formed as intermediates in the cyclo-oligomerization reaction. UV/Vis/NIR titrations show that the three-component assembly of the 12-porphyrin nanoring figure-of-eight template complex displays high allosteric cooperativity and chelate cooperativity. This nanoring–template 1:2 complex is among the largest synthetic molecules to have been characterized by single-crystal analysis. It crystallizes as a racemate, with an angle of 27° between the planes of the two template units. The crystal structure reveals many unexpected intramolecular C–H⋅⋅⋅N contacts involving the tert-butyl side chains. Scanning tunneling microscopy (STM) experiments show that molecules of the 12-porphyrin template complex can remain intact on the gold surface, although the majority of the material unfolds into the free nanoring during electrospray deposition.
Highlights
Ever since Sondheimer’s seminal work on annulenes,[1] macrocycles with p-conjugated perimeters have provided fascinating systems for testing theories of molecular electronic structure
This concept was first illustrated by the synthesis of a 12-porphyrin nanoring c-P12 by coupling a linear porphyrin tetramer l-P4 in the presence of a hexadentate template T6 (Scheme 1).[9d]. Here, we present a full account of the synthesis, crystal structure, and template-binding behavior of c-P12, including an investigation into the mechanism of Vernier templating
During our initial work on the synthesis of the cyclic porphyrin hexamer c-P6tBu, by palladium-catalyzed oxidative coupling of the linear porphyrin monomer l-P1tBu or dimer l-P2tBu in the presence of the hexapyridyl template T6 (Scheme 1a), we noticed the formation of a high-mass byproduct, which was identified as the 12-porphyrin nanoring figure-of-eight complex c-P12tBu·(T6)2.[9b,c] Analytical gel permeation chromatography (GPC) analysis of crude reaction mixtures (Figure 1 a,b) indicated that c-P12tBu·(T6)2 was formed in yields of 9 and 12 % from l-P1tBu and l-P2tBu, respectively
Summary
A. Perdigao,[b] Alex Saywell,[b] Marc Malfois,[c] James N.
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