The compounds (η5-C5H5)2Zr(BH4)2, (η5-C5H5)2Hf(BH4)2 and (η5-(C5H5)2Zr(H)BH4exhibit exchange of C5H5 and BH4 hydrogens which is rapid on the proton NMR time scale at elevated temperatures. The process is found to be predominantly unimolecular, with free energies of activation (Δ Gt) for the above compounds of 21.3 ± 0.4, 19.4 ± 0.4, and 19.6 ± 0.4 kcal/mol (395°K), respectively. For (C5H5)2Zr(H)BF4, the hydrido hydrogen also appears to become involved in the exchange process at higher temperatures, with ηGt = 20.9 ± 0.5 kcal/mol (395°K). The interchange process also occurs for (C5H5)2Zr(BD4)2 in the solid state, but more slowly. The possible mechanisms of this prototropic rearrangement are discussed, and the likely importance of η1-C5H4M carbene-ylid complexes along the reaction coordinate is underscored.
ASJC Scopus subject areas
- Colloid and Surface Chemistry