In-situ spectroscopic of the early events in the Rodium mediated Pauson Khand reaction

Abstract

The current dissertation studied the pre-catalytic events involved in the Rh4(CO)9(A?-CO)3 mediated Pauson Khand reaction using in-situ spectroscopy. In particular, the investigation focused on understanding the (i) catalytic precursor and (ii) its reaction with alkynes. The study of the catalytic precursor Rh4(CO)9(A?-CO)3 allowed the identification of the long-sought all-terminal cluster Rh4(CO)12 spectrum. The formation of various butterfly clusters (alkyne)Rh4(CO)8(A?-CO)2 were observed when reacting equimolar amounts of alkyne with Rh4(CO)9(A?-CO)3. The (3-hexyne)Rh4(CO)8(A?-CO)2 cluster was characterized by mid-infrared, Raman and NMR spectroscopy. In addition, both the deconvoluted Raman and mid-infrared spectra of the butterfly cluster were in good agreement with the DFT predicted spectra. The rate of formation of the (3-hexyne)Rh4(CO)8(A?-CO)2 was Kobs[hexyne][Rh4(CO)9(A?-CO)3][CO]-1 indicating inhibition by higher CO partial pressures. Two further new species were discovered namely (alkyne)Rh2(CO)6 and (alkyne)Rh2(CO)5 after reacting excess alkyne with Rh4(CO)9(A?-CO)3. The rate of the fragmentation of the (1-heptyne)Rh4(CO)8(A?-CO)2 was found to be inhibited by CO as well with rate= Kobs[(1-heptyne)Rh4(CO)8(A?-CO)2][heptyne][CO]-1. Finally, a study of the reaction of the catalytic precursor with enyne resulted in the identification of a new class of butterfly clusters namely (1-heptenyne)Rh4(CO)7(A?-CO)2 with both the alkyne and the alkene moiety from the same substrate coordinating to rhodium.