From para-Benziporphyrin to Rhodium(III) 21-Carbaporphyrins: Imprinting Rh eta super(2)-CC, Rh eta super(2)-CO, and Rh eta super(2)-CH Coordination Motifs

Rhodium(III) para-benziporphyrin alters the fundamental reactivity of the built-in para-phenylene moiety. Due to additional macrocyclic stabilization, a sequence of intramolecular rearrangements are triggered to afford rhodium(III) 21-carbaporphyrin, which incorporates the rhodacyclopropane motif. The peculiar reversible transformations of the bridging methylene unit provide an example of selective and reversible aliphatic C--H bond elimination. Rhodium(III) 21-carbaporphyrin can be oxygenated to rhodium(III) 21-oxy-21-carbaporphyrin, whereas the metal ion interacts with the C(21)--O(25) fragment in an eta super(2)fashion. This species demonstrates a remarkable axial affinity toward alkenes. A playground for exploration: Rhodium(III) para-benziporphyrin undergoes a rhodium(III)-stimulated contraction to afford rhodium(III) mu -methylenecarbaporphyrin and subsequently rhodium(III) 21-oxy-21-carbaporphyrin (see figure). The intermolecular collaboration between the rhodium center and the macrocyclic environment facilitated stabilization of the rhodacyclopropane architecture. The reversible methyl-methylene-hydride transformation provides a snapshot of a selective and reversible C--H bond elimination.