REACTIVITY OF RHODIUM AND IRIDIUM OCTAETHYLPORPHYRINS WITH SMALL MOLECULES: THERMODYNAMIC MEASUREMENTS AND SOLVENT EFFECTS

Rhodium octaethylporphyrin hydride reacts with carbon mon- oxide under ambient conditions to produce a thermodynamically stable metalloformyl complex, reaction 1. (UNFORMATTED TABLE FOLLOWS) Rh(OEP)(H) + CO (DBLHARR) Rh(OEP)(CHO) (1) (TABLE ENDS) A metalloformyl is the first intermediate species in the Fischer-Tropsch reduction of CO. An understanding of the thermodynamics and kinetics involved in the formation of Rh(OEP)(CHO) would facili- tate the development of a homogeneous catalytic cycle as shown in the following scheme. (UNFORMATTED TABLE FOLLOWS) H(,2) (OH)(CH(,2)CH(,2)(OH) M-M 2 M-H (UPARR) (DARR) CO 2 M-CH(,2)(OH) 2 M-CHO H(,2) (TABLE ENDS) In contrast, iridium octaethylporphyrin hydride reacts with carbon monoxide to form a six-coordinate adduct, reaction 2, not the metalloformyl, reaction 3. (UNFORMATTED TABLE FOLLOWS) Ir(OEP)(H)(CO) (2) Ir(OEP)(H) + CO Ir(OEP)(CHO) (3) (TABLE ENDS) Thermodynamic studies of the rhodium and iridium systems were conducted with the use of multinuclear and variable temperature FT-NMR. The metal-metal, metal-carbon and metal-hydrogen bond dissociation energies were measured and related to the various reac- tions of rhodium dimer, rhodium hydride, iridium hydride and iridium methyl complexes. The reactivity of these complexes is drastically affected by a change of the solvent medium. The hydrogen atom of the metallohydride exhibits behavior ranging from acidic to hydridic depending upon the donor properties of the solvent. In addition, both homolytic and heterolytic cleavage of the metal-metal bond in the rhodium and iridium dimers have been observed. These studies have led to a better understanding of the factors that affect the unique reactivities of these systems