Reaction of Alkynes with Rh4(CO)12. A Mid-Infrared Vibrational and Kinetic Study of (μ4-η2-alkyne)Rh4(CO)8(μ-CO)2

Abstract

The pure component mid-infrared spectra of the butterfly clusters (μ4-η2-alkyne)Rh4(CO)8(μ-CO)2, alkyne = 3-hexyne, 1-heptyne, 1-octyne, 4-octyne, 1-phenyl-1-hexyne, and 1-phenyl-1-butyne, were obtained from multicomponent solutions using band-target entropy minimization (BTEM). DFT was used to carry out full geometric optimization and mid-infrared vibrational prediction of (μ4-η2-2-butyne)Rh4(CO)8(μ-CO)2 and (μ4-η2-propyne)Rh4(CO)8(μ-CO)2 as simple models for the terminal and symmetric alkyne clusters, respectively. The predicted spectra were in good agreement with the experimentally obtained deconvoluted pure component spectra of this class of complexes. The kinetics for the formation of the butterfly cluster (μ4-η2-3-hexyne)Rh4(CO)8(μ-CO)2 from the reaction of Rh4(CO)12 with 3-hexyne in n-hexane, i.e., Rh4(CO)12 + 3-hexyne → (μ4-η2-3-hexyne)Rh4(CO)8(μ-CO)2 + 2CO, was also investigated. The rate of formation of the butterfly cluster was found to follow the rate expression, rate = kobs[Rh4(CO)12]1[3-hexyne]1[CO]-1, with the apparent activation parameters ΔH⧧ = 123.6 ± 11.0 kJ/mol·K and ΔS⧧ = (7 ± 4) × 10 J·mol-1·K-1. A mechanism is proposed consistent with the observed kinetic rate expression. This involves a dissociation of one of the carbonyl ligands from Rh4(CO)12 prior to the alkyne coordination.