Spectral resolution of fluxional organometallics. The observation and FTIR characterization of all-terminal [Rh-4(CO)(12)]

摘要

In situ FTIR spectroscopy at 1 cm(-1) resolution was conducted on n-hexane solutions of the bridged [Rh-4(CO)(9)(mu-CO)(3)] in 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 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 [Rh-4(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 = [Rh-4(CO)(12)]/[Rh-4(CO)(9)(mu-CO)(3)] at 275 K was ca. 0.011 and the determined equilibrium parameters were Delta(,)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 [M-4(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.