Scandium arene complexes

CHART 20 Formally zero-valent and subvalent scandium arene complexes. 

Rare earth arene complexes are good precursors for difficult-to-synthesize rare earth compounds (Bochkarev, 2002) and the same was true for the scandium arene complexes supported by NN . Hessen previously showed that a scandium complex of the 2,2 -bipyridyl radical anion could be readily accessed from a reduced 1,3-diene scandium complex (Beetstra et al., 2003). Similarly, the addition of 2,2 -bipyridine to a CeDs solution of Sc2-naph (Scheme 3A) led to the formation of the previously reported radical anionic bipyridyl complex (NN )Sc(2,2 -bipyridine) (Williams et al., 2010). 

The success of direct P4 activation by scandium arene complexes was mirrored when other rare earths, such as yttrium, lanthanum, and lutetium, were employed. The yttrium, lanthanum, and lutetium naphthalene complexes were prepared by a similar protocol to that of Sc2-naph (Scheme 5). Different from... 

In the special case of scandium, another "tuck-in Sc" complex was obtained alongside with the Sc° complex it is proposed that this com-poimd occurs through direct insertion of Sc into a C-H bond of one fBu substituent on the Ar ring prior to arene complexation (Cloke et al., 1991) (Scheme 17). 

Molecules that have [ML/X Z ] classifications that are remote from the highly populated areas in an MLX plot are necessarily of considerable interest because of their uniqueness. For example, with respect to scandium chemistry, while the majority of compounds possess a ML/X3 classification (/ = 2-6), there are some very interesting examples of a zerovalent arene complexes ( j -ArH)2Sc e.g. ArH = C6H3Bu 3) which possess the MLg classification. This corresponds to a most unusual valence state for scandium, and the ability to isolate... 

Scandium Fused-Arene Complexes Synthesis, Characterization, and Reactivity... 

The observed lack of scandium reduced arene complexes may be related to the unique characteristics of scandium. Scandium(III) is the smallest among... 

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. 

This chapter summarized recent advances in the reduction chemistry of rare earth metals and described our own efforts in synthesizing inverse sandwiches of rare earth arene complexes using ferrocene-based diamide ligands. Unprecedented molecules were synthesized and their unusual electronic structures were studied. Highlights included the synthesis of the first scandium naphthalene complex and its reactivity toward P4 activation and the isolation and characterization of a 6-carbon, lOTi-electron aromatic system stabilized by coordination to rare earth metals. The reactivity of those complexes was also discussed. 

Huang W, Khan SI, Diaconescu PL. Scandium arene inverted-sandwich complexes supported by a ferrocene diamide ligand. J Am Chem Soc. 2011 133 10410-10413. 

A novel scandium complex formed from scandium triflate in the presence of 18-crown-6 and p-sulfonatocalix[4]arene has been investigated.224 The crown ether resides in cavities created by two calixarenes from adjacent polymeric sheets. There are two types of scandium ions in the complex, one type is bound to the phenolic oxygen of the calixarene and the second exists as the hydroxide bound dimer ion [Sc(OH)2(H2O)10]4+. A similar complex may also be prepared without the crown ether. 

The trityl borate [Ph3C][B(C6Fs)4] did not cause CeFs back transfer. However, if the coordination site on Sc had sufficient open, the solvent molecules would coordinate to the metal center to form solvent-separated ion pairs (SSIPs), which were inactive in catalysis of polymerizations. A series of scandium methyl SSIPs 124-128 stabilized by the relatively less steric effect P-diketiminato L24 were prepared by reacting 120 with [Ph3C][B(C6Fs)4] in an arene solvent. The synthetic route is shown in Scheme 41 and the complexes were studied for the relevance to propagation processes in the polymerization of alkene. Competition... 

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. 

The parent oxacalix[3] arenas show little ability to bind alkali metals,however, a range of quaternary ammonium cations are attracted to the symmetric cavity. Deprotonation of the phenol moieties allows them to bind to transition metals (scandium, titanium, vanadium, rhodium, molybdenum, gold, etc.), lanthanides (lutetium. yttrium, and lanthanum), and actinides (uranium as uranyl). Oxacalix[3]arenes derivatized on the lower rim can complex gallium, mercury, and alkali metals, including sodium, in a manner reminiscent of natural transmembrane cation filters. One major use was to purify crude samples of fullerenes. The pseudo-Cs symmetry of the macrocyclic cavity is complementary to threefold symmetry elements of Cgo. which binds preferentially in... 

Metal vinyl and aryl complexes are two typical classes of products resulting from sp C—H bond activation from alkene and arene substrates, respectively. Bercaw and coworkers conducted a systematic study of the reactions between Cp 2ScR (R = alkyl or hydride) and various hydrocarbons and found that while ethylene underwent rapid polymerization with Cp 2ScMe, substituted alkenes, such as styrene and isobutene, were readily activated through cr-bond metathesis to result in scandium vinyl compounds (Scheme 8A). One... 

Calixarene complexes with lanthanides have been developed to catalyze the homopolymerization of TMC and DTC. The complexes were synthesized from lanthanide isopropoxide and appropriate p-tcrt-butylcalix[n]arene (n = 4, 6, and 8). Based on the polar solvent effect, the end-group examination of the polymers, and NMR analyses of the growing chain, it could be concluded that the polymerizations of TMC and DTC initiated by p-tcrt-butylcalix[n]arene (n = 4, 6, and 8) complexes of Nd, La, and Y proceeded by a coordination-insertion mechanism. Cou et used scandium p-tcrt-... 

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