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 gamma-cyclodextrin with the binuclear metal carbonyl complex (eta(5)-C5H5)(2)Fe-2(CO)(4) as guest molecule is reported. C-13 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 (eta 5-C5H5)(2)Fe-2(CO)(4). Analysis of the temperature-dependence of the NMR line shapes shows that the microenvironment provided by the gamma-cyclodextrin cavity allows much more extensive dynamic rearrangements of the guest molecules, in comparison to pure cis- or trans- (eta(5)-C5H5)(2)Fe-2(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 environment, and the rates of various exchange processes are usefully modified compared to those detected in solution. For (eta(5)-C5H5)(2)Fe-2(CO)(4) included in gamma-cyclodextrin, contrary to the situation found in solution, the rate of bridging-terminal carbonyl exchange in the cis isomer is greater than the rate of cis-trans isomerization; in solution direct bridging-terminal exchange in the cis isomer could not be studied because indirect exchange via isomerization to the trans form, which undergoes rapid bridging-terminal exchange, is always significantly faster. By restricting isomerization, the inclusion environment thus confirms for the first time that the cis isomer is capable of carbonyl exchange and would allow the study of its rate and activation parameters.