Deoligomerization

In a similar way, a set of disilanyl and polysilanyl complexes has recently been synthesized and exposed to photochemical deoligomerization reactions [137]. The photolytically obtained reactive silylene complexes have been identified by trapping experiments [138, 139]. 

A few heteroleptic complexes have been isolated in the presence of bulky deoligomerizing ligands [15,77,78], e.g., CpfLa(NHCH3)(H2NCH3) [77] or Cp Sm(HNPh) [65b]. 

In contrast, Nd2(OCHiPr2)6 cannot be deoligomerized by these solvents, including pyridine. This is in accord with the lower nucleophilicity of the siloxide O-atom which is reflected in the less distinct tendency to bridge the oxophilic lanthanide centers [20]. 

A number of authors have reported significant increases in the volatility of Ba(tmhd)2 (prepared by both hydrous and anhydrous methods) and fluorinated /3-diketonate compounds when they are heated in the presence of bases such as thf, NH3, pyridine, and Htmhd.164,218,219 The increase in volatility has been attributed to deoligomerization of the [Ba5(thd)9(H20)3 (OH)], [Ba4(tmhd)8], and [Ba(/3-diketonateF)2] aggregates by these bases. This is supported by isolation of compounds such as [Ba(thd)7 Et20]2, [Ca(hfac)2(OH2)2]2, and [Ba(tmhd)2(NH3)2]2.153,169,174... 

Scheme 1. Deoligomerization of Fp-containing polysilanes. (Reproduced from reference 12. Copyright 1986 American Chemical Society.)...

Photolysis of bulky permethylated Fp complexes such as FpSi[Si(CH3)3]3 does not cause deoligomerization, possibly because stable intermediate iron-silyl(silylene) complexes are not formed (27). Other less bulky transi-tion-metal-oligosilane complexes are also unreactive under the photolysis conditions. For example, the ruthenium analogues of the iron complexes, [( ri -C5H5)Ru(CO)2-Si ], are essentially photostable (23). Whether this behavior is due to the strength of the Ru-CO bond or to the enhanced stability of the Si-Si bond is not clear, and this problem is currently under investigation. 

Finally, for the Fe complexes, a direct Fe-Si bond is required to activate the oligomers. When the silyl group is bonded to the complex via the cy-clopentadienyl ring, as in [(7i -(CH3)3Si(CH3)2Si(CH3)2Si(C5H4)-Fe(CO)2CH3], photolysis does not lead to deoligomerization (12). 

An excellent review was quite recently reported on achiral additives as a poison to kill an undesired catalyst species and/or to deoligomerize less active catalysts Vogl, E.M. Groger, H. Shiba-saki, M., Angew. Chem., Int. Ed. Engl., 1999, 1570-1577. 

Figure 3.8.2 The electronic spectrum for (n-Bu2Sn) showing photochemical deoligomerization. The final spectrum is shown by the-----dotted line. (Adapted from Reference 13.)...

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