One-Electron Activation of CO by a Rhodium(II) Porphyrin Bimetalloradical Complex and Concerted Reactions of Two (RhCO)• Units

A rhodium(II) porphyrin bimetalloradical complex, • Rh(CH 2 ) 6 Rh • ( 1 ) reacts with CO and ethene to form an ethanedionyl reductive coupling product, Rh(CH 2 ) 6 Rh(μ-C(O)C(O)) ( 2 ), and an ethylene-bridged complex, Rh(CH 2 ) 6 Rh(μ-CH 2 CH 2 ) ( 4 ), respectively. Reaction of 1 with 1,3-butadiene proceeds by a 1,4-addition to yield a four-carbon-bridged complex, Rh(CH 2 ) 6 Rh(μ-CH 2 CH CHCH 2 ) ( 5 ). 1 H NMR spectra of 2 and 4 manifest the inherent chirality of two-carbon-bridged tethered diporphyrin complexes. Solutions of 1 and CO react with H 2 O, CH 3 CH 2 OH, and H 2 to produce multifunctionalized formyl complexes H Rh(CH 2 ) 6 Rh CHO ( 6 ), H(O)C Rh(CH 2 ) 6 Rh C(O)OCH 2 CH 3 ( 7 ), and H(O)C Rh(CH 2 ) 6 Rh CHO ( 8 ), respectively, but fail to react to an observable extent with methane. Reactivity studies indicate that solutions of 1 and CO function as a source of a species that contains two carbonyl groups ( • [OCRh(CH 2 ) 6 RhCO] • ) ( 3 ) that are activated toward one-electron reactions at the carbonyl carbon. Reactions of 3 with substrates (H 2 O, C 2 H 5 OH, H 2 ) are discussed in terms of a proposed pathway where two metalloformyl radicals ([MCO] • ) and the substrate occur in a four-centered transition state (Rh C(O) • ···A···B··· • (O)C Rh).