Stereochemically non-rigid species

The NMR spectroscopic examples discussed so far have assumed that, with the exception of free rotation about single bonds, the molecule or ion is static in solution. For the majority of organic and inorganic species, this assumption is valid, but the possibUily of stereochemical non-rigidity (fluxionality) on the NMR spectroscopic timescale must be considered. Five-coordinate species such as Fe(CO)5, 4.10, PF5,4.11, and BrF5,4.12, constitute one group of compounds for which the activation barrier for dynamic behaviour in solution is relatively low, and exchange of substituent positions is facile. 

The molecular stanctuie of [Gd(DTPA-BMAXOH2)] (Omniscan) detnmined by X-iay diffiactian [A. Aiikiust et al. (2001) Org. Proc. Res. Develop., vol. 5, p. 361]. Colour code  

The iron oxide nanoparticles are coated in polysaccharides (dextran or carboxydextran) to make them hiocompatihle and are administered to patients as aqueous colloids. Clinical MRI contrast agents of this type include Feridex, Resovist and Combidex which are selectively taken up hy the liver, spleen and bone marrow. The S5mthesis and potential applications in MRI of related iron oxide nanoparticles is currently an active area of research. 

Accardo, D. Tesauro, L. Aloj, C. Pedone and G. Morelli (2009) Chem. Rev., vol. 253, p. 2193 - Supramolecular aggregates containing lipophiUc Gd(III) complexes as contrast agents in MRI . 

Hermann, J. Kotek, V. Kubrcek and I. Lukes (2008) Dalton Tram., p. 3027 - GadoUnium(III) complexes as MRI contrast agents Ugand design and properties of the complexes . 

The NMR spectroscopic examples discussed so far have assumed that, with the exception of free rotation about single bonds, the molecule or ion is static in solution. For the majority of organic and inorganic species, this assumption is valid, but the possibility of stereochemical non-rigidity (fluxionality) on 

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Although F, Cl, Br and I all possess spin active nuclei, in practice only F (100%, / = j) is used routinely. Fluorine-19 NMR spectroscopy is a valuable tool in the elucidation of structures and reaction mechanisms of F-containing compounds see case studies 1 and 5 and the discussion of stereochemically non-rigid species in Section 3.11. 

Group IV Donors. A H n.m.r. study of the cis-trans isomerization of [OslCO) -(SiMe3)2] indicates that the process is non-dissociative. These species may therefore provide a rare example of stereochemical non-rigidity in nonchelate octahedral complexes. Similar non-rigidity was observed for [Os(CO)4-(SiMeCl ) ] and [Os(CO)4(SnMe3)2]. ... 

The [MH(PF3)J complexes (M = Co, Rh, Ir) and the isoelectronic species [M H(PF3)4] (M = Fe, Ru, Os) are all stereochemically non-rigid, and detailed multinuclear NMR spectroscopic studies have been carried out and activation parameters determined. It has been proposed that the phosphorus environments are interchanged by a tetrahedral tunnelling rearrangement mechanism. Some typical temperature-dependent and 19F NMR spectra are shown in Fig. 7. 

Solution NMR studies are more plentiful still, and underpinned by knowledge of the crystal structures, basic geometries and axial or equatorial site preferences may often be deduced. Many pentacoordinated species are stereochemically non-rigid in view of the significance of this topic a separate section is devoted to it. 

Several famillies of organometallic oxides have been described and especially the first carbene derivative of polyoxometallates. The concept of isolobality between some oxo and organometallic fragments has been extended to Ru(arene) - containing species, which could turn out to be convenient precursors for catalytic purposes. Stereochemical non rigidity has been evidenced in the system [M40]6 Ru(arene) 4], which is under further investigation. 

Scheme 23.35 implies that the intermediate is a coordina-tively unsaturated species. In the presence of a solvent, S, such a species would probably be stabilized as Mn(CO)4(COMe)(S). In the absence of solvent, a 5-coordi-nate intermediate is likely to be stereochemically non-rigid (see Figure 2.13 and discussion) and this is inconsistent with the observation of a selective cw-relationship between the incoming CO and acyl group. It has been concluded from the results of theoretical studies that the intermediate is stabilized by an agostic Mn—H—C interaction (structure 23.37), the presence of which locks the stereochemistry of the system. ... 

A large fraction of the known 5-coordinate species have structures that are appreciably distorted from one or other of the two prototype structures in some cases the distortion is such that it becomes uncertain which of the two ought to be taken as the idealized geometry. This ready deformability is characteristic of 5-coordination and closely connected with the dynamic stereochemical non-rigidity so prevalent among these structures. 

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