Fluxionality of Cp₂Mo₂(CO)₆
In 1973, Adams, Brice and Cotton demonstrated that in Cp₂Mo₂(CNMe)(CO)₅ there is CNMe transfer between the molybdenum atoms, and by implication, carbonyl exchange.
Cp₂Mo₂(CO)₆ exists in solution as a mixture of trans and gauche isomers. In most solvents, the predominant isomer is the trans. In 1989, Lindsell and Tomb showed that the ratio can be reversed by working in a mixture of DMSO and THF as the solvent. We have exploited this observation and used DMF as the solvent to study carbonyl exchange in ¹³CO enriched Cp₂Mo₂(CO)₆.
The lowest energy dynamic process is carbonyl exchange in gauche-Cp₂Mo₂(CO)₆. This results in carbonyls C¹O and C³O exchanging via an intermediate gauche-Cp₂Mo₂(µ-CO)₂(CO)₄.
The next lowest dynamic process is the corresponding carbonyl exchange in in trans-Cp₂Mo₂(CO)₆. This results in carbonyls C¹O and C³O exchanging via an intermediate trans-Cp₂Mo₂(µ-CO)₂(CO)₄. This type of intermediate has previously been characterised for trans-Cp₂Mo₂(µ-CNCF₃)₂(CO)₄ by Lentz and co-workers.
The highest energy dynamic process is rotation about the Mo-Mo bond. As was previously observed by Lindsell and Tomb, only interconversion between the trans and gauche isomers was observed. The barrier for gauche-gauche is too high to detect. This is presumably due to the difficulty in rotating one cyclopentadienyl group past the other.
Scheme. Rotation of the Mo-Mo bond in Cp₂Mo₂(CO)₆. Viewed along the Mo-Mo bond with the carbonyls trans to the Mo-Mo bond omitted for clarity.
R. D. Adams, M. Brice, and F. A. Cotton, J. Am. Chem. Soc., 1973, 95, p. 6594.
X. Chen and B. E. Mann, Organometallics, 1996, 15, p. 3703.
D. Lentz, I. Brudgam, and H. Hartl, J. Organomet. Chem., 1986, p. 299, C38.
W.E. Lindsell and P. J. Tomb, J. Organomet. Chem., 1989, 378, p. 3703.