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

Article abstract of DOI:10.1039/b500044k

In situ FTIR spectroscopy at 1 cm^-1 resolution was conducted on n-hexane solutions of the bridged [Rh₄(CO)₉-(μ-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 [Rh ₄(CO)₁₂] species with a C_3v anticubeoctahedron structure where 2 different [Rh(CO)₃] moieties exist, although the presence of some T_d structure can not be entirely excluded. The equilibrium between all-terminal [Rh₄(CO)₁₂] and the bridged [Rh₄(CO)₉(μ-CO)₃] was determined in the presence of both helium and CO. The equilibrium constant K_eq = [Rh₄(CO)₁₂]/[Rh₄(CO)₉-(μ-CO) ₃] at 275 K was ca. 0.011 and the determined equilibrium parameters were Δ_rG = 12.63 ± 4.8 kJ mol^-1, Δ_rH = -21.45 ± 2.3 kJ mol^-1 and Δ_rS = -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 Royal Society of Chemistry 2005.