Numerical Treatment of In-Situ Data for the Pre-Catalytic Transformations of Rhodium Carbonyl Species and Implications

Abstract: In-situ FTIR spectroscopy at 1 cm resolution was conducted on n-hexane solutions of Rh4(σ-CO)9(μ-CO)3 on the interval T=268-288 K and PT=0.1-7.0 MPa using either helium or carbon monoxide as dissolved gas. Analysis of the spectral data sets was conducted using BTEM in order to recover the pure component spectra. A new spectral pattern was recovered with terminal vibrations at 2041, 2043, 2069.8 and 2075 cm-1. The new spectrum indicates the presence of either one all terminal structure Rh4(σ-CO)12 with quasi T symmetry, or the co-existence of two highly symmetrical terminal geometries with similar free energies. The equilibrium between Rh4(σ-CO)12 and Rh4(σ-CO)9(μ-CO)3 was determined in the presence of both helium and CO. The equilibrium constant K = [Rh4(σ-CO)12]/[Rh4(σ-CO)9(μ-CO)3] at standard conditions was circa 0.011 and the determined equilibrium parameters were ∆rG = 12.63 ± 4.8 kJ / mole, ∆rH = -21.45 ± 2.3 kJ/mole and ∆rS = 114.3 ± 8.35 J/mole K. As anticipated, the free energy difference is very low, and the unbridged isomers have a lower entropy – consistent with a higher symmetry. This finding helps to address a long-standing issue concerning the existence of various M4(σ-CO)12 symmetries. Pure component spectra of other known rhodium complexes, namely, Rh2(σ-CO)6(μ-CO)2 and Rh6(σ-CO)12(μ3-CO)4 were also recovered. No conclusive evidence for the postulated species Rh2(σ-CO)8 nor Rh4(σ-CO)8(μCO)4 could be obtained. In-situ FTIR spectroscopy at 1 cm resolution was conducted on n-hexane solutions of Rh4(σ-CO)9(μ-CO)3 on the interval T=268-288 K and PT=0.1-7.0 MPa using either helium or carbon monoxide as dissolved gas. Analysis of the spectral data sets was conducted using BTEM in order to recover the pure component spectra. A new spectral pattern was recovered with terminal vibrations at 2041, 2043, 2069.8 and 2075 cm-1. The new spectrum indicates the presence of either one all terminal structure Rh4(σ-CO)12 with quasi T symmetry, or the co-existence of two highly symmetrical terminal geometries with similar free energies. The equilibrium between Rh4(σ-CO)12 and Rh4(σ-CO)9(μ-CO)3 was determined in the presence of both helium and CO. The equilibrium constant K = [Rh4(σ-CO)12]/[Rh4(σ-CO)9(μ-CO)3] at standard conditions was circa 0.011 and the determined equilibrium parameters were ∆rG = 12.63 ± 4.8 kJ / mole, ∆rH = -21.45 ± 2.3 kJ/mole and ∆rS = 114.3 ± 8.35 J/mole K. As anticipated, the free energy difference is very low, and the unbridged isomers have a lower entropy – consistent with a higher symmetry. This finding helps to address a long-standing issue concerning the existence of various M4(σ-CO)12 symmetries. Pure component spectra of other known rhodium complexes, namely, Rh2(σ-CO)6(μ-CO)2 and Rh6(σ-CO)12(μ3-CO)4 were also recovered. No conclusive evidence for the postulated species Rh2(σ-CO)8 nor Rh4(σ-CO)8(μCO)4 could be obtained. 1.0 Introduction The group VIII (Co, Rh, Ir) tetranuclear metal carbonyl clusters M4(CO)12 have been the subject of intense research for over 40 years. In particularly, their structures and fluxional behavior in solution has generated considerable discussion. The rhodium cluster Rh4(σ-CO)9(μ-CO)3 represents a special case due in part to its highly fluxional behavior. The x-ray structure has been determined on at least 3 occasions, and an idealized C3v symmetric icosahedral has been shown. In solution, the infrared spectra are consistent with the same geometry at room temperature. In addition, the C NMR and Rh solution spectra are also consistent with the bridged C3v symmetric icosahedral geometry at least in the vicinity of 213 K. However, at circa 268 K coalescence occurs indicating equivalence of all CO on the NMR time scale. This suggests the inter-conversion: C3v ↔ all-terminal ↔ C3v. However, to date the presence of the all terminal species have not been confirmed. In the present contribution the solution chemistry of Rh4(CO)12 is re-investigated under various helium or CO partial pressures in n-hexane as solvent on the interval T=268-288 K and PT=0.1-7.0 MPa using FTIR as the spectroscopic tool and BTEM as the signal processing algorithm. BTEM has repeatedly shown its ability to retrieve pure component spectra from very weak signal intensities, i.e. for HRh(CO)4. 2.0 Results and Discussion BTEM successfully resolved the pure component spectra of Rh6(CO)16, Ni(CO)4 and Fe(CO)5 (impurities from gas cylinders).and their time dependent concentration were obtained. In addition and for the first time the fragmentation of the Rh4(σ-CO)9(μ-CO)3 to Rh2(σ-CO)6(μ-CO)2 was unequivocally observed at mild temperatures and relatively low pressures. Along with the upper species, a new spectral pattern was recovered, specie X in figure 1. After careful inspection of the new spectral, it was assigned to the long sought after Rh4(σ-CO)12. Figure 1. Pure component spectra obtained from BTEM analysis. The normalized concentrations of the solutes are automatically provided by the BTEM analysis and are shown in figure 2. Figure 2. The normalized metal carbonyl concentrations provided by BTEM analysis for the 7 semibatch reactions. Normalized concentration allows detailed explanation of each of species concentration profiles. In addition, comparison between the behavior of each specie under carbon monoxide versus