Reaction Intermediates in the Solution Photochemistry of a Tetrarhodium Carbonyl Cluster:  FTIR Structural Characterization and Transient Kinetics

The photochemistry of the tetranuclear rhodium carbonyl cluster, Rh4(CO)12, in heptane solution has been studied using pulsed laser photoexcitation coupled to time-resolved step-scan FTIR spectrometry. It was found that excitation of the cluster with 266 nm light results in CO dissociation to form a Rh4(CO)11(Solv) species, transformations of which were observed in the nanosecond time domain. Monitoring the reaction course revealed the presence of two successive transients, I and II, which eventually reform the parent cluster. The FTIR spectrometric study showed that these transients represent two isomeric forms of the Rh4(CO)11(Solv) species. The second transient exhibits the structure typical of ground-state substituted Rh4(CO)11(L) derivatives with the solvent molecule in an axial position of the basal plane of the metal tetrahedron. The first isomer is a “nonequilibrium” form of the same species where the solvent molecule occupies either a radial site at the basal rhodium triangle or a vacancy at the apical rhodium atom. The kinetics of the reaction sequence is consistent with a unimolecular isomerization of I into II and trapping of the latter by CO evolved at earlier stage of reaction. In the presence of excess CO in the reaction mixture the first transient is effectively scavenged to form Rh4(CO)12. The kinetic parameters of the isomerization process and the trapping of I by CO have been determined.