Self-Assembly of Zinc Porphyrins around the Periphery of Hydrogen-Bonded Aggregates That Bear Imidazole Groups

This paper describes the preparation and characterization of four aggregates that are based on the rosette of derivatives of isocyanuric acid (CA) and melamine (M). These aggregates comprise a trismelamine, hub(MIm)3, that presents imidazole groups around its periphery; these imidazoles organize zinc tetraphenyl porphyrin (ZnTPP) by coordination of the imidazole to the zinc center. Aggregate 1 forms a single rosette upon mixing 1 equiv of hub(MIm)3 and 3 equiv of CA. Adding 3 equiv of ZnTPP yields 2. Aggregate 3 forms as a stacked bisrosette upon mixing 2 equiv of hub(MIm)3 and 3 equiv of bisCA. Adding 6 equiv of ZnTPP yields 4. The stoichiometries of aggregates 1−4 were obtained by titrating the trismelamines with CA and by titrating the aggregates with ZnTPP. The stoichiometry is defined as the ratio at which additional CA remains insoluble or additional ZnTPP appears as free ZnTPP in the 1H NMR spectrum. Electrospray ionization mass spectrometry (ESI-MS) is compatible with the measured stoichiometries. The structures of these aggregates were determined using variable-temperature 1H NMR spectroscopy; analogous structures were inferred for 5 and 6, the tert-butyl analogues of 1 and 2. The shapes of the traces from gel permeation chromatography (GPC) suggest that imidazole groups destabilize the aggregates when they are not involved in coordination to zinc; that is, the stability seems to be 6 ≈ 4 > 3 and 5 ≈ 2 > 1. A direct comparison of the relative stability of 1, 2, and 5 confirms the results of the GPC analysis: mixing 1 (hub(MIm)3·3CA) with the trismelamine component of 5 (hub(M)3) leads to the formation of a 3:2 mixture of 5:1. Adding ZnTPP to this solution leads to a 3:2 mixture of 5:2 with free trismelamines remaining in solution: 1 is not observed. The results of UV/vis spectroscopy are consistent with the other spectroscopic and chromatographic results and indicate that 3 equiv of ZnTPP are organized around the periphery of 2 and at least 4 equiv around the periphery of 4.