A 13C CP/MAS NMR Study of the Structure and Dynamics of [(η5-C5H5)2Fe2(CO)4] Included in γ-Cyclodextrin: Evidence for Terminal−Bridging Exchange in the cis Isomer
The novel inclusion compound of γ-cyclodextrin with the binuclear metal carbonyl complex (η5-C5H5)2Fe2(CO)4 as guest molecule is reported. 13C CP/MAS NMR spectroscopy, in the temperature range 100 to 353 K, is used to probe the structure and dynamics of the included molecules. Specifically, below ca. 240 K evidence is presented for the existence of both cis and trans isomers of included (η5-C5H5)2Fe2(CO)4. Analysis of the temperature-dependence of the NMR line shapes shows that the microenvironment provided by the γ-cyclodextrin cavity allows much more extensive dynamic rearrangements of the guest molecules, in comparison to pure cis- or trans-(η5-C5H5)2Fe2(CO)4, for which no isomerization or bridging−terminal carbonyl exchange processes are observed in this temperature regime. Notably, even at 100 K, bridging−terminal carbonyl exchange for the included trans isomer is rapid on the exchange-broadening time scale. However, the inclusion cavity is still more dynamically restrictive than a solution environme...