Encapsulation transition metals

Figure 8.4 Modeled structure of an encapsulated transition metal catalyst, consisting of zinc(ii)TP) (gray), tris(meta-pyridyl)phosphine (3) (white) and M = [HRh(CO)3] (dark-gray). The different pyridylphosphine template-ligands used for assembly processes are also shown.

A. W. Kleij, M. Lutz, A. L. Spek, P. W. N. M. van Leeuwen, J. N. H. Reek, Encapsulated transition metal catalysts comprising peripheral Zn(ll)salen building blocks template-control-led reactivity and selectivity in hydroformylation catalysis, Chem. Common., 2005, 3661. 

Slagt VF, et al. Assembly of encapsulated transition metal catalysts. Angew Chem Int Ed 2001 40 4271-4. 

Blum J., Avnir D., Schumann H. Apphcations of sol-gel encapsulated transition metal catalysts. Chemtech. 1999a 29 32-38... 

Rosenfeld A., Avnir D., Blum J. Sol- el encapsulated transition metal quaternary ammonium ion pairs as highly efficient recyclable catalysts. J. Chem. Soc., Chem. Commun. 1993 583 584 Rosenfeld A., Blum J., Avnir D. Disproportionation of dihydroarenes by soL el entrapped RhCls-quaternary ammonium ion pair catalysts. J. Catal 1996 164 363-368 Sarussi L., Blum J., Avnir D. Doped sol gel materials as heterogeneous reagents for organic synthesis. J. Sol-Gel Sci. Tech. 2000 19 17-22... 

Rosenfeld, A., Avnir, D., and Blum, J. (1993) Sol-gel encapsulated transition metal quaternary ammonium ion pairs as highly efficient recyclable catalysts. /. 

The encapsulation of classical and organometallic transition-metal complexes to yield molecules of the type complex in a complex is a very attractive research area. A variety of inclusion complexes of this type has been reported. Of relevance for this review are reports on the encapsulation of coordinatively unsaturated transition-metal complexes inside self-assembled coordination cages (120), cyclodextrins (121,122), and cucurbiturils (123). 

The encapsulation of transition-metal complexes within the cavity of 24 has not yet been reported. 

Martin et al. have developed a unique series of capped tris(l,2-di-aminoethane) cages which can encapsulate divalent transition metal ions in a near octahedral geometry (28). The iron(II) complex with the ligand (NH2)2sar turns out to be a crossover system in solution [34], but the solid triflate salt is low-spin [43]. This is the only Fe(II) crossover system having 6 identical aliphatic nitrogen donors. 

The trend was studied and verified, for instance, for reactions catalysed by transition metal, organo- and enzyme catalysts entrapped in ORMOSIL prepared by copolymerization of tetramethoxysilane (TMOS) and the modifying co-precursor methyltrimethoxysilane (MTMS). It has been correlated with the encapsulation itself but also with the structure of the sol-gel matrix, namely the hydrophobicity-lipophilicity balance (HLB) and the textural properties of the materials.9... 

Related to these dimetallic systems, though not involving transition metals, are the boratastilbene complexes such as a " [ H 5 C 5 B-CH =CH-CH 4 -C H =C H P h ]" (isoelectronic with distyrylbenzene chromophores) that show aggregation-dependent photophysics. In nonpolar solvents, they form tightly bound ion pairs that are poorly luminescent, but in polar solvents, or when the counter ions are encapsulated in crown ethers, strong emission is observed as a result of intramolecular charge transfer.130... 

Partially encapsulated Tl+ ions have been observed for the related doubly bridged compound 107.103 The transition metal fragment [ Me2Pt 2(/r-Ph2PC5H3NPPh2)2] is first assembled and then treated with TICIO4 to give the product in 60% yield. The Tl+ ion was displaced by Cu+ upon addition of [Cu(MeCN)4]C104 almost quantitatively. 

Endohedral doping (encapsulation) of other materials within carbon nanostructures can be carried out by nano-capillary effects or during synthesis (Fig. 4.3(b)). A great variety of halides, oxides, metals and alloys have been encapsulated within CNTs [36-41]. When transition metals are encapsulated, the entire sample can exhibit high magnetic coercivities ca. 0.22 T [42,43]. The encapsulation of C60 molecules can also be accomplished and if the material is heat treated at high temperatures... 

---