Synthesis and X-ray structure of the rhenium methyl complex trans-Cp*Re(CO)(2)(Me)I and a study of the products of photolysis of the rhenium alkyl methyl and dimethyl complexes Cp*Re(CO)(2)(Me)R (R = Ph, p-tolyl, Me) under CO

Abstract

Reaction of Cp*Re(CO)(2)I-2 with methylcopper affords cis-Cp*Re(CO)(2)(Me)I, which converts to the trans isomer on prolonged reaction or in the presence of neutral alumina. The X-ray structure of the trans isomer has been determined. The related chloro complexes Cp*Re(CO)(2)(Me)Cl and Cp*Re(CO)(2)(p-tolyl)Cl are formed in the photolyses of compounds 3 and 1 (below) in CCl4. Photolysis of Cp*Re(CO)(2)(Me)R (R p-tolyl (1), Ph (2), Me (3)) in the presence of CO has been carried out in hydrocarbons, CCl4, and benzene-d(6). In hydrocarbons, 1 and 2 produce Cp*Re(CO)(3), CH4, and either toluene or benzene, respectively; 3 produces Cp*Re(CO)(3) and CH4. In benzene-d(6) 1 gave CH3D and toluene-4-d, and 3 gave mainly CH3D. These results are consistent with a general scheme involving successive homolysis of the metal-methyl and metal-aryl bonds to give methyl and aryl radicals that abstract H or D from the solvent and carbonylation of the rhenium dicarbonyl fragment. Products known or expected to arise from further photolysis of Cp*Re(CO)(3) in benzene-d(6), such as Cp*Re-2(2)(CO)(3), Cp*Re-2(2)(CO)(5), and Cp*Re(CO)(2)(eta(2)-C6D6), were also found. Photolysis of 1 in CCl4 in the presence or absence of CO gave CH3Cl and Cp*Re(CO)(2)(p-tolyl)Cl, but no p-chlorotoluene, indicating the preferential homolysis of the Re-Me bond and the rapid scavenging of the subsequent radicals by the chlorinated solvent. Photolysis of the dimethyl complex 3 gave CH3Cl and some evidence of a small amount of Cp*Re(CO)(2)(Me)Cl, but the major rhenium product was Cp*Re(CO)(2)Cl-2, consistent with the more facile homolysis of both Re-Me bonds in 3. Production of small amounts of CH2D2 (in benzene-d(6)) and CH4 and CH2Cl2 (in CCl4) are discussed in terms of a competing pathway. Notably, in none of these photolyses were there observed other than trace amounts of products such as p-xylene, which would be expected to be major products if reductive elimination were to occur.