Thermal ligand exchange

In this review our focus will be largely on thermal ligand exchange and isomerizations. Other reactions, including photochemical ones, will be dealt with only briefly. Most examples will come from studies of cobalt(III) and chromium(III) compounds merely because the chemistry of these metals is more fully developed. 

The stability of adducts 146, 147, and 150 was supported by thermal gravimetric analysis (DSC/TGA) ill-defined mass-loss begins at 200 °C. On the other hand, no ligand exchange was observed with r/s pyridine (C5D5N) up to 200 °C in toluene solutions. 

The catalytic activation of thionyl chloride by Bi(0Tf)3-xH20 has been reported and applied to the preparation of the thermally unstable aryl sulfinyl chlorides [34]. It has been shown that this procedure is restricted to electron-rich aromatics (such as aromatic ethers and mesitylene). The mechanistic investigation seems to indicate that no ligand exchange occurs during the reaction. 

The flash data cannot, in my opinion, require such a process, since even a very inefficient reaction (27) might be expected—especially in view of the great sensitivity of this system to thermal exchange119—to give rise to large amounts of ligand exchange. Furthermore, thermal sensitivity... 

Much less is known about photoinitiated ligand exchange reactions of coordination complexes of cobalt(III) and other de ions than is the case for chromium(III). With the exception of the cobalt(III) ammines, however, available data suggest that photochemical ligand exchange reactions of cobalt(III) and d6 systems involve the thermally equilibrated 17 g and/or 37, states of the complex as intermediates. The reasoning is completely analogous to that described earlier with respect to chromium(III) photochemistry. 

Despite the kinetic lability of the Ln-X-cr-bonds (even the thermodynamically very stable Ln-OR bond is subject to rapid ligand exchange reactions [49]) organolanthanide compounds are thermally very robust over a wide range of temperature (Fig. 5) [114, 116, 139, 144-151]. Thermal stability is not only favorable in catalytic transformations at elevated temperatures [47], for the support of volatile molecular precursors is of fundamental importance in chemical vapor deposition techniques the sublimation behavior is a criterion of thermal stability and suitability for these processes (Fig. 5). 

Similar chemistry occurs with R3SnCo(CO)4 if the reaction is performed photochemi-cally in hexane however, when conducted thermally in polar solvents, cobalt fluorides, R3SnF and cobalt cluster compounds are formed342. In the case of an attempted insertion into a Rh—Sn bond under photochemical conditions, ligand exchange chemistry occurred (equation 146)343. 

When 92 is heated with C6D6 at 80°C, a ligand exchange reaction occurs. The thermal reactions of 92 with toluene, ortho-, meta-, and para-xylene, and mesitylene have been monitored by NMR spectroscopy. In all cases... 

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