Rhodium Complexes of Carbaporphyrins, Carbachlorins, adj-Dicarbaporphyrins, and an adj-Dicarbachlorin

The macrocyclic cavities in carbaporphyrins are well suited for the formation of metalated derivatives. A carbaporphyrin diester and a naphthocarbaporphyrin reacted with [Rh(CO)2Cl]2 to give good-to-excellent yields of rhodium(I) complexes, and these were fully characterized by X-ray crystallography. Both rhodium(I) derivatives were converted into rhodium(III) complexes in refluxing pyridine, albeit in moderate yields. Carbachlorins also formed rhodium(I) complexes, but these could not be further transformed into rhodium(III) products. The rhodium(III) complexes incorporate two axial pyridine ligands, which exhibit strongly shielded resonances in their 1H NMR spectra, and the rhodium(III) carbaporphyrin diester was further characterized by X-ray crystallography. adj-Dicarbaporphyrins also formed rhodium(I) complexes, but these reactions involved the relocation of a proton to generate an internal methylene unit. The environments associated with the two faces of the resulting macrocycles are very different from one another, and this results in the 1H NMR chemical shifts for the two internal methylene protons being separated by well over 3 ppm. Although the diatropicities of rhodium(I) complexes for monocarbaporphyrins and carbachlorins are comparable to those of the parent ligands, the chemical shifts for rhodium(I) dicarbaporphyrins are consistent with a significant reduction in the porphyrinoid aromaticity. A dicarbachlorin also gave a rhodium(I) complex, but this species fully retained the diatropic characteristics of the parent ligand. Nevertheless, the internal CH2 unit still gave two widely separated doublets indicative of radically differing environments for the two faces of the macrocycle. Rhodium(I) dicarbaporphyrin and dicarbachlorin complexes were further characterized by X-ray crystallography.