Determining the pure component spectra of trace organometallic intermediates by combined application of in situ Raman spectroscopy and band-target entropy minimization analysis

Trace organometallic intermediates arising from complex organic syntheses are usually quite difficult to detect spectroscopically. In situ FTIR and in situ NMR are the only techniques that are used with any regularity for such studies. In this contribution, high-pressure in situ Raman spectroscopic measurements were performed for the rhodium catalyzed hydroformylation of 3,3-dimethylbut-1-ene using Rh4(CO)12 as catalyst precursor at 298 K – a reaction extensively studied previously by more sensitive in situ FTIR. The Raman spectroscopic measurements were analyzed using the band-target entropy minimization (BTEM) algorithm. As expected, the pure component spectra of dissolved CO, 3,3-dimethylbut-1-ene, and 4,4-dimethylpentanal were easily recovered. In addition, the pure component spectra of the precursor Rh4(CO)12 and the intermediate RCORh(CO)4 (R = (CH3)3CCH2CH2) were successfully reconstructed – even though the mean concentrations of both species were on the order of 150 ppm. The BTEM estimate of the Raman spectrum of RCORh(CO)4 is reported for the first time. This Raman spectrum is consistent with the DFT predicted spectrum. This study represents the first combined application of Raman spectroscopy and BTEM analysis to a homogeneously catalyzed metal-mediated reaction. The potential and limitations of this general approach are discussed.