Scandium divalent complexes

Further treatment of Sc(P3C2But)3 with KCg in toluene at low temperature (—78 °C) produced a very deep blue solution, the color of which persisted at room temperature. Sublimation of the residue after removal of the volatiles gave very dark blue crystals of [Sc(P3C2But)2] containing formally divalent scandium. The complex is dimeric in the solid state and should best be described as a mixed oxidation state complex containing both Sc1 and Scm centers.733,73311... 

Tml2, and that of other isolated uncommon divalent complexes there are also two endohedral metallofullerenes Sc Cg4 (Takahashi et al., 1995) and Tm C82 (Kirbach and Dunsch, 1996 Pichler et al., 1997) for which spectroscopic evidence points to the divalent nature of Sc and Tm, respectively they are worth mentioning here but they will not be discussed further. We will also consider the important issue of chemical systems in which uncommon divalent species may be present or have been postulated as intermediates, or displaying divalent-like reactivity. We will also report on the few low-valent scandium species that have been reported and finally on the zero-valent compounds. 

In the specific case of scandium, mono- and mixed-valence species can also be isolated together with divalent complexes for instance, a Sc organometallic compoimd could be obtained imder relatively mild conditions. Finally, the author describes the few knovm zerovalent bis(arene) rare-earth complexes which have been obtained by co-condensation of arenes or heteroarenes with metal vapors. In his conclusion, F. Nief notes that the low-valence molecular chemistry of rare earths, which was once thought to be restricted to divalent samarium, europium, and ytterbium, has been extended to several other rare earths, as well as to lower valence oxidation states. It is the opinion of the author that this research area is likely to find fascinating developments in a near future. 

In the very recent past, metal complex catalysis has been used with advantage for the stereo- and enantio selective syntheses based on the Henry and Michael reactions with SENAs (454-458). The characteristic features of these transformations can be exemplified by catalysis of the reactions of SENAs (327) with functionalized imides (328) by ligated trivalent scandium complexes or mono-and divalent copper complexes (454) (Scheme 3.192). Apparently, the catalyst initially forms a complex with imide (328), which reacts with nitronate (327) to give the key intermediate A. Evidently, diastereo- and enantioselectivity of the process are associated with preferable transformations of this intermediate. 

As we shall see, there are now other rare-earth elements that are stable enough in the divalent state in solution, allowing for the successful isolation of molecular complexes that can be fully characterised, including structurally, and their reactivity studied. Recent reviews and highlights have partially addressed this topic (Bochkarev, 2004 Cassani et al., 2002 Evans, 2000, 2002a,b, 2007 Izod, 2002 Meyer, 2008). Eurthermore, there are a few reports on low-valent scandium complexes, and also a number of extremely reactive zero-valent rare-earth molecular compounds that could only be made via metal vapour techniques, which have been reviewed some time ago (Cloke, 1993). 

The reaction of Sc2-naph with excess pyridine (Scheme 3B) led to the isolation of a rare, reductively 4,4 -C-C-coupled diamide linker that bridges the two scandium ions in the complex [(NN )Sc(NC5H5)]2[p-(NC5H5-C5H5N)]. A similar reduction was reported in the reactions of divalent thulium complexes (Fedushkin et al., 2003 Jaroschik et al., 2007b) and, recently, samarium(ll) (LabouiUe et al., 2012) with pyridine however, the reaction of a rare earth arene complex with pyridine had not been reported previously. 

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