Spectral resolution of fluxional organometallics. The observation and FTIR characterization of all-terminal [Rh4(CO)12]

Abstract

In situ FTIR spectroscopy at 1 cm(-1) resolution was conducted on n-hexane solutions of the bridged [Rh4(CO)9(mu-CO)3] in the interval T= 268-288 K and P(T)= 0.1-7.0 MPa using either helium or carbon monoxide as dissolved gas. Analysis of the spectral data sets was conducted using band-target entropy minimization (BTEM), in order to recover the pure component spectra. A new spectral pattern was recovered with terminal vibrations at 2075, 2069.8, 2044.6 and 2042 cm(-1). The new spectrum is consistent with an all-terminal [Rh4(CO)12] species with a C(3v) anticubeoctahedron structure where 2 different [Rh(CO)3] moieties exist, although the presence of some Td structure can not be entirely excluded. The equilibrium between all-terminal [Rh4(CO)12] and the bridged [Rh(4)(CO)9(mu-CO)3] was determined in the presence of both helium and CO. The equilibrium constant K(eq)=[Rh4(CO)12]/[Rh4(CO)9(mu-CO)3] at 275 K was ca. 0.011 and the determined equilibrium parameters were Delta(r)G= 12.63 +/- 4.8 kJ mol(-1), Delta(r)H=-21.45 +/- 2.3 kJ mol(-1) and Delta(r)S=-114.3 +/- 8.35 J mol(-1) K(-1). The free energy indicates a very small difference between the bridged and terminal geometry, and the lower entropy is consistent with a higher symmetry. This finding helps to address a long-standing issue concerning the existence of various [M4(CO)12] symmetries. In a more general context, the present study illustrates the considerable utility of quantitative infrared spectroscopy (occurring on a fast vibrational timescale) combined with sophisticated deconvolution techniques in order to resolve systems which have been demonstrated to be fluxional on the NMR timescale.

The reactions of Rh2(CO)4Cl2 with 1,5-cyclooctadiene and tetramethylallene. A combined in-situ vibrational spectroscopies, spectral reconstruction and DFT study

“…2023 cm À1 was reconstructed during the reaction of TMA with Rh 2 (CO) 4 Cl 2 . The two BTEM estimates are not consistent with the impurities Fe(CO) 5 , Ni(CO) 4 , and Mo(CO) 6 which have sometimes been detected in our other in-situ spectroscopic studies [21]. Due to the low concentrations of these two minor species, their FIR and low wavenumber MIR spectra could not be obtained and therefore their identification is not presently possible.…”

The Catalytic Binuclear Elimination Reaction: Importance of Non-linear Kinetic Effects and Increased Synthetic Efficiency

“…In solution, fluxionality of all COs was observed, and this leads to numerous experimental and theoretical studies. The long sought all-terminal variety of Rh 4 (CO) 12 was not identified until after the advent of the latest generation of signal processing techniques [55]. The dinuclear Rh 2 (CO) 8 is also non-isolatable and was first observed under hundreds of bar CO partial pressure [56].…”

Spectral Renormalization for Multi-Component Fourier Transform Infrared Absorbance Data: Importance to Multivariate Calibration and Quantitative Exploratory Chemometrics Studies

“…[26][27][28] This algorithm is based on a spectral entropy minimization objective and was proven to be an efficacious numerical tool that is able to elucidate the pure component spectral profiles of unknown chemical species with minute spectral signal contributions. 33,38,40 Comparison studies have demonstrated BTEM to be generally superior in resolving pure component spectra from multi-component systems with highly overlapping FT-IR bands. 26,28 One notable application of the BTEM algorithm was the successful elucidation of the long sought after rhodium tetracarbonyl hydride HRh(CO) 4 spectrum, 33 which is the first spectroscopic proof of its existence under hydroformylation conditions of reaction temperature T ¼ 285-303 K, partial pressures P H2 ¼ 1.0-5.0 MPa, P CO ¼ 1.0-5.0 MPa, and initial moles of catalyst precursor § [Rh 4 (CO) 12 ] ¼ 58-315 mg in n-hexane solvent.…”