Alkali metal alkoxides structures

Copolymerization Initiators. The copolyineri/ution of styrene and dienes in hydrocarbon solution wilh alkyllithium initiators produces a tapered block copolymer structure because of the large differences in monomer reactivity ratios lor styrene (r, < 0.11 and dienes (rj > 10). In urder to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such us tetrahydrofuran or an alkali metal alkoxide (MtOR. where Ml = Na, K. Rb, or Cs>. 

Single metal and mixed metal oxo alkoxides can also be prepared by reaction between metal halides, oxy halides, and alkali metal alkoxides.47,48 One of the first to be structurally characterized was a series of mixed Cd or Sn containing zirconium, tin, and titanium alkoxides.84,85 These and other examples are shown in skeletal form in Figure 3. 

The chemistry and structures of alkali metal alkoxides [125], enolates [126], and phenoxides [127] has been compiled in several comprehensive review articles. Simple alkali metal alkoxides such as methoxides, ethoxides, or isopropoxides, and simple thiolates (LiSMe, NaSMe, KSMe, etc.) crystallize in layered structures. Interesting supramolecular self-assembly is found for other base-free and solvated alkali metal alkoxides, phenoxides, and their sulfur analogs. Typical examples are summarized in Table 6.3. 

Polymeric self-assembly has frequently been observed in the chemistry of alkali metal alkoxides and related compounds. Not discussed here are simple alkoxides and thiolates such as MOMe (202, M = Li-Cs) [162-167], MOPr (203, M = K-Cs) [168], and MSMe (204, M = Li-K) [169]. They form layered structures in the... 

C. Andrews, J.G. MacLellan, R.E. Mulvey and P.J. Nichols, New homo- and hetero-alkali metal alkoxide cages Crystal structures of [MejN(CH2)20Li]g and [ Me3N(CH3)30 12Li3KJ0, Dalton Trans., vol. 8, pp. 1651-1655, 2002. 

Bains" reported lithium teri-butoxide to be hexameric in cyclohexane. There appears to be some controversy in the literature about the molecular weight of lithium fert-butoxide in benzene, the reported degree of polymerization being 4.0," and 6.0." " ° The last value appears to be most reliable as these investigators" have confirmed their findings by mass spectrometry. X-ray structural findings have revealed that tetrameric cubanes and hexameric stacks are the most prevalent structural motifs for molecular alkali metal alkoxides. 

Interest in the synthesis and structural characterisation of less common metalloorganophosphide systems has continued. A range of alkali metal complexes of the phosphide anion (48) has been prepared from the lithio-phosphide by simple metathesis with alkali metal alkoxides. The lithio-phosphide exists as a dimer, but crystallisation of the sodium and potassium drivatives in the presence of TMEDA results in the formation of monomeric species. The caesium triphosphenide (49), involving a phosphorus analogue of the allyl anion, has been prepared from the previously described sodium complex. Unlike the sodium and potassium complexes, which are monomeric, the caesium complex has a polymeric structure. A variety of arylphosphide complexes of magnesium, calcium, strontium and barium have been prepared and their solid state structures characterised. Cop-per(I)-, silver(I)- and gold(I)-complexes of the cyclo-(P ion have also... 

Initiation of the EO polymerization by alkali metal alkox-ides has also been investigated in detail since the structure of these derivatives is close to that of alkoxide propagating species. In aprotic solvents of low-to-medium polarity, such as ethers, alkali metal alkoxides show a strong tendency to aggregate and yield complex reaction kinetics. This is particularly significant for small metals such as lithium or sodium. If the reactions follow a monomer order of 1, the order in alkoxide propagating species varies according to the counterion and... 

The structures of organotin amines and alkoxides are tetrahedral, as expected. Organotin alkoxides and phenoxides are prepared from reaction with alcohols and alkali metal alkoxides or phenoxides with the organotin halide. Most of the published reactions occur in the liquid phase. For instance, the dimethoxide is formed from reaction of dibutyltin dichloride in methanol at 0°C in 98% yield. ... 

Mixed carbanion/alkoxide complexes were formed from combining n,sBu2Mg, TuOM (M = Na or K), and TMEDA to form the dimeric CIPs [(Bu)2(tBuO)MMg(TMEDA)]2 (M = Na and K) 437, 438.442 While the two structures are identical in their atom connectivity, they are not precisely isostructural in that K shows a bias toward C-coordination while Na is inclined toward N. The alkali metal cations are both formally five-coordinate bonding with two a-C(Bu) atoms, two N(TMEDA) atoms, and a single 0(cBu) atom. The dominant feature in both structures though is the [(Bu)2Mg(/x-tBuO)2MgBu2]2 dianion. 

Heterobimetallic clusters (Figure 58,125 and 126) with solvent-dependent structures were also obtained upon mixing alkali metal tert-butoxides and -trimethylsiloxides in THF, TMEDA, and toluene.184 The common occurrence of heterocubes shows that there is a strong driving force for the formation of heterocubic structures in organozinc alkoxides. Solvent effects are important, however, as demonstrated by the formation of seeo-diheterocubic compounds in TMEDA. 

Our preliminary attempts to obtain a basic chiral rare earth complex have led us to create several new chiral heterobimetallic complexes which catalyze various types of asymmetric reactions. The rare earth-alkali metal-tris(l,f-bi-2-naphthoxide) complexes (LnMB, where Ln = rare earth, M = alkali metal, and B = l,l -bi-2-naphthoxide) have been efficiently synthesized from the corresponding metal chloride and/or alkoxide,13,41 and the structures of the LnMB complexes have been unequivocally... 

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