Rhodium(II) porphyrin bimetalloradicals: Activation of methane and carbon monoxide

A series of diporphyrin ligands that contain spacers with different degrees of flexibility were synthesized from trimesityl 4-hydroxyphenyl porphyrin derivatives which have steric demands similar to that of tetramesitylporphyrin (TMP). The dirhodium(II) derivatives were obtained by photolysis of the dimethyl complexes and were found to occur as stable bimetalloradical complexes by virtue of the ligand steric demands which prohibit Rh(II)-Rh(II) bonding. The rhodium(II) porphyrin bimetalloradical complexes can activate methane and hydrogen under very mild conditions to form the hydridomethyl and the dihydride complexes respectively at rates that are much faster than the corresponding reactions by (TMP)Rh$\bullet$ and ((TXP)Rh) $\sb2.$ The observed rate enhancements for the bimetalloradical complexes illustrate the kinetic advantage that occurs by preorganizing two non metal-metal bonded metalloradical units in one molecular species such that the substrate reactions can proceed via a bimolecular pathway by the proposed four centered transition state. Rhodium(II) porphyrin bimetalloradical complexes can activate the C-H bonds in a variety of alkanes under mild conditions with unusual features. The reactions have a high substrate selectivity with preference for the smallest molecule, and a different regiospecificity with preference for primary C-H bonds. The high preference for methane over the methanol reaction is especially significant in terms of the selective conversion of methane to methanol which is both fundamentally and practically important. Rhodium(II) porphyrin bimetalloradical complexes can activate carbon monoxide to selectively form the dirhodium $\alpha$-diketone diporphyrin complexes at room temperature. The resulting complexes provide an equilibrium source for an activated metalloorganic diradical complex that contains two metalloacyl radicals in a single molecule. The metalloorganic diradicals have the thermodynamic and kinetic capability to accomplish