Heterobimetallic 21,23-dimetallaporphyrin: activation of metal-metal interactions within the porphyrinoid macrocycle

Two core-modified porphyrins containing metal atoms, namely platinum( ii ) or platinum( iv ) and rhodium( iii ), in place of two NH units, have been obtained by a post-synthetic modification of the 21,23-ditelluraporphyrin. The products of the tellurium-to-metal exchange, 21-platina-23-rhodaporphyrins, incorporate rhodacyclopentadiene and platinacyclopentadiene units with the metal atoms facing each other. The two molecules exhibit different degrees of metal-metal interaction depending on the oxidation state of platinum, with the NBO bond order being 0.04 for platinum( iv ) and 0.15 for platinum( ii ). Consistently, the Quantum Theory of Atoms in Molecules analysis revealed the presence of the bond determinant, the bond critical point, in the platinum( ii ) species, in contrast to the platinum( iv ) congener. The two porphyrinoids are interconvertible in redox reactions. They both exhibit fluxional behaviour in solution, studied by 1 H NMR, involving alteration in the metal ion coordination sphere accompanied by the macrocyclic skeleton conformation change. The incorporation of two metal atoms, platinum and rhodium, in place of NH groups in the porphyrin macrocycle brings them into close proximity. The weak metal-metal bond is switched on and off depending on the oxidation state of the platinum ion.