Carbon-13 nuclear magnetic resonance studies of rhodium carbonyl clusters on pressurization with CO/H2

Abstract

Carbon-13 n.m.r. measurements on rhodium carbonyl clusters in solutions under high pressures of gaseous CO/H₂ are reported. Pressurization of solutions of [Rh₁₂(CO)₃₀]^2– with CO (5 bar) shows that [Rh₅(CO)₁₅]^– is formed quantitatively and that, below 298 K, further fragmentation does not occur on pressurization up to 1 kbar with CO/H₂. Variable-temperature measurements on [Rh₅(CO)₁₅]^– show that the rate of inter-exchange of carbonyls with carbon monoxide at high pressure is slow on the n.m.r. time-scale and that, at room temperature, all the carbonyls, except the three bridging the Rh₃ equatorial plane, undergo intra-exchange. At high ¹³C enrichments, the resonance due to the equatorial bridging carbonyls becomes complex due to an increasing abundance of isotopomers, Rh₃(µ-¹²CO_eq.)_{3 –x}(µ-¹³CO_eq.)_x(x= 0–3), with larger values of x for which ²J(C_eq.–C_eq)= 20 Hz. Despite claims to the contrary, the apex-connected, double-octahedral metallic skeleton in [Rh₁₂(CO)₃₀]^2– remains intact up to 323 K under a nitrogen atmosphere. The high-resolution n.m.r. probe for measurements on solutions under 1 kbar pressures of CO and H₂ gases is described.