SYNTHESIS AND REACTIVITY OF MACROCYCLIC RHODIUM COMPLEXES RELEVANT TO CATALYTIC PROCESSES

A challenging goal of inorganic chemistry involves understanding the role transition metals play in catalyzing the reduction of carbon monoxide to useful organic products. The ability of the rhodium porphyrin system to form strong metal-carbon bonds permits its unprecedented ability to react with H{dollar}sb 2{dollar} and CO gases to form a metallo formyl complex (Reaction 1). (1) (Por)Rh--Rh(Por) + H-H + CO {dollar}rightarrowatopleftarrow{dollar} 2 (Por)Rh--C(O)H. As a consequence of the strong rhodium-carbon bond, a series of thermodynamically stable {dollar}alpha{dollar}-hydroxyalkyl complexes have been prepared by the direct reaction of (Por)Rh(H) with aldehydes (Reaction 2). (2) (Por)Rh--H + R-C(O)-H {dollar}rightarrowatopleftarrow{dollar} (Por)Rh--Ch(R)OH (R = -H, -CH{dollar}sb 3{dollar}, -CH{dollar}sb 2{dollar}CH{dollar}sb 3{dollar}, -(CH{dollar}sb 2)sb 2{dollar}CH{dollar}sb 3{dollar}, -(CH{dollar}sb 2{dollar}){dollar}sb 3{dollar}CH{dollar}sb 3{dollar}). Thermal and photochemical reactions of the isolated metallo {dollar}alpha{dollar}-hydroxymethyl are depicted in reaction 3-5. (3) 2 (OEP)Rh-CH{dollar}sb 2{dollar}OH {dollar}rightarrowatopleftarrow{dollar} ((OEP)Rh-CH{dollar}sb 2{dollar}-) {dollar}sb 2{dollar}O + H{dollar}sb 2{dollar}O; (4) (OEP)Rh-CH{dollar}sb 2{dollar}OH + CO {dollar}to{dollar} (OEP)Rh-C(O)CH{dollar}sb 2{dollar}OH; (5) (OEP)Rh-CH{dollar}sb 2{dollar}OH + (OEP)Rh-H {dollar}to{dollar} (OEP)Rh-Rh(OEP) + CH{dollar}sb 3{dollar}OH.; Dehydration of ethylene glycol to acetaldehyde is catalyzed by the dehydrase enzyme in conjunction with the vitamin B{dollar}sb {lcub}12{rcub}{dollar} cofactor. The strong metal-carbon bond formed with the Rh(Por) system permits the preparation of a {dollar}alpha,beta{dollar}-dihydroxyethyl complex by the direct reaction of (OEP)Rh(H) with gycolaldehyde. Thermal reactions results in an intramolecular condensation reaction to yield a {dollar}beta{dollar}-formyl species (Reaction 6). (6) (OEP)Rh--CH(OH)CH{dollar}sb 2{dollar}OH {dollar}buildrelDeltaoverrightarrow{dollar} (OEP)Rh--Ch{dollar}sb 2{dollar}C(O)H + H{dollar}sb 2{dollar}O.; The unusual ability of the aromatic Rh(Por) system to model many key organometallic intermediates has been attributed to the strong rhodium-carbon bond. A dimeric rhodium complex, containing a single metal-metal bond, has been prepared with the TMTAA ligand. Reactions of this dimeric complex with mixtures of hydrogen and carbon monoxide gases results in the formation of a metallo hydride and formyl species (Reaction 7 and 8). (7) (TMTAA)Rh--Rh(TMTAA) + H-H {dollar}rightarrowatopleftarrow{dollar} 2 (TMTAA)Rh--H; (8) (TMTAA)Rh--Rh(TMTAA) + H-H + 2 CO {dollar}rightarrowatopleftarrow{dollar} 2 (TMTAA)Rh--C(O)H.; In summary, key organometallic intermediates have been prepared and their reactivity investigated, with the rhodium porphyrin system relevant to vitamin B{dollar}sb {lcub}12{rcub}{dollar} and Fischer-Tropsch catalyzed reactions. Additionally, the ability of the Rh(TMTAA) system to react with mixtures of hydrogen and carbon monoxide indicates that the unusual reactivity of these systems may be available to a wide range of structurally analogous metal dimers. (Abstract shortened with permission of author.)...