Stereochemical nonrigidity of [Rh6(CO)15L] clusters in solution

Abstract

A series of substituted hexarhodium carbonyl clusters [Rh₆(CO)₁₅L] (L = PR₃ (R = alkyl, aryl), P(OPh)₃, NCMe, I⁻) has been studied by variable temperature and two-dimensional, X-{¹⁰³Rh}, (X = ¹³C, ³¹P) HMQC and ¹³C EXSY NMR spectroscopy in solution. At low temperatures, the spectra are consistent with retention of the solid state structure. Different localised exchanges of terminal (CO_t) and face-bridging (CO_fb) CO's are found to occur over different atoms of the Rh₆-octahedron at higher temperatures and the different pathways of the exchanges are discussed. When L = PR₃ (R = alkyl, aryl), the lowest energy scrambling surprisingly involves exchange of CO_t and CO_fb, associated with the substituted rhodium (S-type), with concomitant exchange of L between the two terminal sites on the substituted rhodium, followed by other localised stereospecific exchanges involving CO's associated with unsubstituted rhodium atoms (U-type). For the other substituted clusters (L = P(OPh)₃, NCMe, I⁻), only U-type exchanges are observed. The kinetics of these exchanges are reported at different temperatures and for the S-type exchange mechanism, the rate is found to vary with the nature of PR₃.