Intramolecular Host–Guest Dynamics of FeCp(CO)₂X (X = I and CH₃) and Mo₂Cp₂(CO)₆ Included in β‐ or γ‐Cyclodextrin

Abstract

Variable‐temperature 13C CPMAS NMR spectra of the metal–carbonyl complexes FeCp(CO)2X (where X = I, CH3) and Mo2Cp2(CO)6 included in β‐ or γ‐cyclodextrin (CD) cavities were investigated in the temperature range 133 to 293 K. The motion of the guest molecule inside the CD can be detected by comparing these spectra with the static crystalline patterns of FeCp(CO)2I and Mo2Cp2(CO)6. In the case of FeCp(CO)2CH3, solid‐state plastic crystal behaviour is noted for the free guest; the molecular motion is further modified by inclusion within the CD cavity. The nature of the molecular motion found within the metal–carbonyl/CD adducts was shown to be dependent on the symmetry, size and orientation of the guest molecule within the host cavity. In the case of Mo2Cp2(CO)6 included in γ‐CD, the two [MoCp(CO)3] ends of the dimer may be considered as separate dynamic entities: one half of the moiety within the CD cavity exhibits a greater freedom of motion, whereas the other end of the dimer is anchored. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Solid-state NMR studies of weak interactions in supramolecular systems

“…A particular case of mobility inside a CD cavity is represented by the Mo 2 Cp 2 (CO) 6 -b-CD adduct which shows more unusual results with respect those presented above. 80 At first glance the ambient temperature 13 C CPMAS spectrum of Mo 2 Cp 2 (CO) 6 included in g-CD does not differ greatly from the spectrum recorded for the polycrystalline specimen. Furthermore the spinning sideband pattern associated with the carbonyl resonances looks quite similar in the two spectra.…”

“…Another case is represented by FeCp(CO) 2 I. 80 In the crystalline FeCp(CO) 2 I the static nature of the carbonyls is demonstrated by the presence of a large manifold of spinning sidebands associated with the isotropic resonances at 211.3 ppm. The analysis of their tensor components affords a Ds value of 411 ppm.…”

Solid‐State NMR Investigation of Ligand Mobility and Reactivity in Transition Metal Complexes

“…[67] Fast motion of one (η 5 -C 5 H 5 )-Mo(CO) 3 part inside the CD cavity has been suggested, MHz with a spinning speed of 10 kHz. [67] whereas the other half part of the dimer non-included in the CD cavity appears rigid due to the presence of strong interatomic interaction with neighbouring molecules.…”

Solid‐State NMR Studies on Supramolecular Chemistry

“…Interestingly the VT 13 C SSNMR spectra of (η 5 -C 5 H 5 ) 2 Mo 2 (CO) 6 included in γ -CD can only be interpreted by fast motion of one half of the dimer inside the CD cavity, whereas the nonincluded (η 5 -C 5 H 5 )Mo(CO) 3 part is rigid due to strong interatomic interaction with neighboring molecules. 69 Fast motion of the (η 5 -C 5 H 5 )Mo(CO) 3 part inside the hydrophobic cavity is explained by the weaker constrain forces.…”