Niobium aryloxides

Aromatics. A new generation of homogeneous arene hydrogenation catalysts was developed by Rothwell and coworkers. These are hydride derivatives of niobium and tantalum with bulky ancillary aryloxide ligands (see, i.e., 43) exhibiting high regio- and stereoselectivities 467... 

The unsubstituted para-t-butyl calixarenes themselves complex metals via their aryloxide groups. Since aryloxide complexes are frequently oligomeric, the simple calixarenes do not give monomeric complexes. Aryloxides are hard ligands, therefore they readily form complexes with oxo-philic hard metal ions such as alkali metals, early transition metals, lanthanides, and actinides. Complexation is often inferred because the calixarene acts as a carrier for the metal ion from an aqueous to an organic phase. With the /wa-/-butylcalix[ ]arenes in alkaline solution, a value of n = 6 gives the best carrier for lithium(I), sodium(I), and potassium(I), with a value of n 8 giving the best carrier for rubidium(I) and caesium(I).15,16 Titanium(IV) complexes have been characterized,17-19 as well as those of niobium(V) and tantalum(V).20-22 These complexes are classified as... 

A very different set of interesting d niobium and tantalum hydride catalysts containing bulky aryloxide ligands recently reported by Rothwell [52-54] appear very promising. For instance, naphthalene and anthracene are reduced at 80 C and 3-100 atm Hj by [Ta(OC H3(C H,)2-2,6 2(H)j(PMc,Ph)] to produce mainly tetralin or 1,2,3,4-tetrahydroanthracene, respectively. 

In other systems, N—cleavage gives bis(/i-nitrido)dimetal diamond cores as the products of N2 cleavage. A reduced niobium complex of calix[4]arene reduces N2 in the presence of sodium metal, and a number of intermediates can be isolated. A related tridentate aryloxide ligand also splits N2 to give a diamond core with lithium ions bound to the nitrides. A vanadium diamidoamine complex reacts with N2 to give a product with a similar core structure, which may be further reduced by potassium graphite to a compound with one unpaired electron. This... 

Tsukasa M, Hiroyuki K. Synthesis and structures of niobium(V) complexes stabilized by linear-linked aryloxide tiimers. Inorg Chem 2002 41(23) 6090-8. 

Table 6.1 Metal-ligand bond lengths for mixed alkyl, aryloxides of niobium and tantalum...

A series of 1,3-cyclohexadiene derivatives have also been obtained by reduction of chloro, aryloxides of niobium and tantalum." These compounds are potential intermediates within the catalytic cycle of arene hydrogenation. Structural smdies of derivatives such as [M(OAr)3( /" -C6H8)] (M = Nb, Ta OAr = OC6H3Pi -2,6) show a metallanorbornene bonding picture. These compounds are not only precursors for arene hydrogenation, but will also catalyse both disproportionation and hydrogenation of 1,3-cyclohexadiene." ... 

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