The genesis of carbon-supported FeMn and KFeMn catalysts from stoichiometric metal carbonyl clusters: I. Characterization by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)

Abstract

The thermal decomposition of Fe 3(CO) 12, NEt 4[Fe 2Mn(CO) 12], Mn 2(CO) 10, K[HFe 3(CO) 11], and K[Fe 2Mn(CO) 12] has been studied for the first time by dispersing these clusters on an oxygen-free carbon surface and monitoring their behavior by diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). The Fe 3(CO) 12 decomposed to Fe(CO) 5 in either He or H 2, while Mn 2(CO) 10 decarbonylated without the formation of any stable intermediate clusters in either gas. The NEt 4[Fe 2Mn(CO) 12] clusters decomposed with no formation of other stable carbonyl products in He, but in H 2 they formed Mn 2(CO) 10 and [HFe 4(CO) 13] − species. Similarly, the decomposition of K[Fe 2Mn(CO) 12] in He produced no detectable intermediates, but under H 2 it led to the formation of these same two intermediate clusters, while K[HFe 3(CO) 11] yielded [HFe 4(CO) 13] −. First-order rate constants of decomposition were determined for each cluster, compared to literature k 1, values for nucleophilic substitution reactions in solution, and found to be similar to substitution rate constants for Fe 3(CO) 12 and Mn 2(CO) 10 but higher than those for Fe(CO) 5. The rate-determining step in either the substitution or the decomposition reaction appears to be the removal of the first CO ligand in Fe 3(CO) 12, but with Mn 2(CO) 10 it is MnMn bond scission. The activation energies of decomposition were 18–21 kcal/mol for Fe 3(CO) 12 and 32–40 kcal/mol for Mn 2(CO) 10, while those for the decomposition products gave intermediate values. This study represents a portion of the first successful application of an IR spectroscopic technique to characterize carbon-supported metal catalysts.