Rhenium and technetium porphyrin complexes with unusual geometries

Abstract

Monometallic and bimetallic porphyrin complexes of rhenium and technetium nave been synthesized by reaction of stoichiometric quantities of Re₂(CO)�₀ or Tc₂(CO)�₀ with porphyrin In refluxing decalin under argon. The monorhenium porphyrin complex can further react with either Re₂(CO)�₀ or Tc₂(CO)�₀ to form the homo- and hetero-dinuclear metalloporphyrin complexes. A single-crystal X-ray diffraction analysis indicates that the dirhenium porphyrin is a centrosymmetrlc complex having two rnenium atoms bonded to the porphyrin, one above and one below the plane of the macrocycle. The porphyrin macrocycle is highly distorted. The metal ions are not positioned directly over the center of the macrocycle, but are set to one side such that each metal is bonded to three nitrogen atoms. The ditechnetium porphyrin is isostructural with the dirhenium porphyrin. Although an X-ray structure has not yet been done on any of the monometallic complexes, their chemical and physical properties, which are close to that of the bimetallic species, indicate a similar structure. The proton magnetic resonance spectrum of (H-TPP)Re(CO)₃ also supports the proposed structure of the monometallic porphyrin complexes. The rhenium and technetium monometallic porphyrin complexes have fluxional character as seen by variable-temperature par spectra studies. This fluxional phenomenon is best explained by the intramolecular rearrangement of the metal-carbonyl group among the four porphyrinato nitrogen atoms and a concomitant movement of the N-H proton. An unusual thermal disproportionation of (H-MP)Tc(CO)₃ to MP[Tc(C0₃)]₂ and H₂MPIXDME is also observed for the monotechnetium porphyrin complex. A dissociation and recombination of the metal-carbonyl moieties and the porphyrin ligands between two of the (H-MP)Tc(C0)₃ molecules explains this phenomenon..