Group 14 hydrides, polymerization

All the elements of Group 13 (boron group) form polymeric hydrides (MH3)j . The monomers MH3 are strong Lewis acids and are unstable. Borane (BH3) achieves... 

Hydrides can be categorized in various classes according to the nature of the bond that links the metal and hydrogen. The following categories can usually be found in the literature ionic hydrides (hydrides with group 1 and 2 elements), covalent hydrides, polymeric hydrides, metallic hydrides, borderline hydrides, and transitirai metal complex hydrides. Here the reaction of ionic hydrides with carbon dioxide (CO2) is considered. Transition metal hydrides with the formula L M-H are discussed in Chap. 4, where the interaction of CO2 with analogous E-X systems (where E and X are different elements) is discussed. 

Treatment of thiiranes with lithium aluminum hydride gives a thiolate ion formed by attack of hydride ion on the least hindered carbon atoms (76RCR25), The mechanism is 5n2, inversion occurring at the site of attack. Polymerization initiated by the thiolate ion is a side reaction and may even be the predominant reaction, e.g. with 2-phenoxymethylthiirane. Use of THF instead of ether as solvent is said to favor polymerization. Tetrahydroborates do not reduce the thiirane ring under mild conditions and can be used to reduce other functional groups in the presence of the episulfide. Sodium in ammonia reduces norbornene episulfide to the exo thiol. 

The addition of monomer to the latter anion leads to the polymer, In this procedure the reaction of sec-BuLi with the C = 0 group is prevented, and due to the very low concentration of the reactive CH2C(CH3) (CO OCH3), Li+ their disproportionation, presumably involving hydride ion transfer, is minimized also. All these factors contribute to the cleanness of polymerization. 

Our attempts to prepare chromium hydrides and to evaluate their role in polymerization catalysis eventually led to the isolation of a series of alkyls and hydrides lacking any ancillary ligands besides the cyclopentadienyl moiety (see below).[6] Reduced to the essence of alkyls, these complexes provided another piece of evidence in the growing case against polymerization activity of divalent chromium none of the alkyls even reacted with ethylene. The hydride underwent one insertion and stopped at the stage of an ethyl group. 

Hydride complexes of Group IV metallocenes have been implicated as catalysts and as important intermediates in olefin hydrogenahon and polymerization reactions [232, 233]. 

Further studies quickly revealed that the rapid dehydrogenative coupling of primary organosilanes to oligomers and the slower coupling of secondary silanes to dimers can be effected under ambient conditions with compounds of the type CP2MR2 (M = Ti, R = alkyl M = Zr, R = alkyl or H)(11,12,13). None of the other metallocenes, metallocene alkyls, or metallocene hydrides of groups 4, 5 or 6 have shown any measurable activity for polymerization... 

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