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

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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-12COeq.)3 –x(µ-13COeq.)x(x= 0–3), with larger values of x for which 2J(Ceq.–Ceq)= 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.

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