Cubane structure alkoxides

The formation of tetrameric aggregates with a hetero-cubane structure is a structural motif that is observed for several alkylzinc alkoxides in the solid state (Figure 80). In these structures the alkoxy group is -bonded with its oxygen atom to three zinc atoms. The structural elucidation of [MeZnOMe]4 (152) represented the first example of such a structural motif in organozinc alkoxide chemistry . Later, similar structures were observed for alkylzinc alkoxides containing other alkoxy groups (153, = f-... 

More complicated structures of alkylzinc alkoxides have been observed in the solid state with the stoichiometry R eZn7(OR )8. These compounds have a hetero-di-cubane structure (two hetero-cubanes sharing a corner), and have been observed for Me6Zn7(OMe)g (159)240,241 Et6Zu7(OMe)8 (160)242 (ICH2)6Zu7(OMe)8 (161)2 (Figure 80). 

FIGURE 80. Alkylzinc alkoxides having a hetero-cubane structure... 

AUcoxide complexes of stoichiometry [Ni(OR)2] can be prepared by reaction of simple NF salts NiX2 with RO. In general, the stmctures of these complexes are not known for certain, although the X-ray stracture of [Ni4(OMe)4(acac)4(MeOH)4] (76) confirms it to be based on a Ni4(OMe)4 cube. Similar cubane structures have been proposed for a variety of other nF alkoxide complexes. 

The amide method was found to be convenient and straightforward in many cases. An example is the synthesis of soluble barium terr-butoxide, which has a cubane structure (106b). The preparation of this compound by other routes (e.g, direct action of metals with alcohols or chloride-alkoxide interchange), sometimes tends to be complicated due to the incorporation of chloride or oxide moieties within the structural framework of the final alkoxide product. 

The cubane structure [M4(m3-OR)4] (Fig. 4.6) is favoured by the alkali metal tertiary butoxides (M = K, Rb, Cs) and thallium(i) alkoxides, and also related species such as [Me4Zn4(/ t-OMe)4]. Another common tetranuclear structure [M4(jti-OR)6(OR)6] (Fig. 4.7) involves one central octahedral metal bridged to three tetrahedral metals in a chiral configuration (D3). Crystalline aluminium isopropoxide was the first example established to have this structural type. ° Interestingly in the heterometallic complex [ErAl3(OPr )i2] the central position is occupied by the erbium ion in a distorted trigonal prismatic configuration. ... 

In addition, crystal structures of several halomethylzinc alkoxides and an acy-loxymethylzinc reagent have been reported (Figure 2, 7-9). lodo- and chloromethylzinc alkoxides exist as tetramers in cubane or bis(cubane)-like structures. However, these... 

Thallium(I) ethoxide has been known for many years, and a series of related alkoxides can be prepared by methods which include Tl + ROH, TIOH + ROH and T120 + ROH.295 The ethoxide,296 and a number of other TIOR compounds,297 are tetrameric in solution, and an X-ray crystallographic study of (T10Me)4 shows that the structure is based on that of cubane,298 with no T1—T1 bonding interactions. The bonding in such molecules has been discussed.299 Phenoxide, and substituted phenoxides, can be prepared from PhOH + TlOH, while reaction... 

Figure 1 Prototypal structures for small oligomeric metal alkoxides (a) the cubane-M404 unit in [TKOMe) , and [Na(OBu )]4 (b) die planar [CufOBu1) , structure (c) and (d) fused trigonal and fused trigonal-tetrahedral units as seen in [Be(OR)2) compounds, where n = 3 and 2 (e) fused tetrahedral-octahedral units seen in [Al(OPri)3]4 (f) edge-shared tetrahedra as in [Al(OBu )3]2 (g) fused octahedra as in [MfOEt), , where M = Ti, V, W and (h) edge-shared octahedra as...

The tetranuclear iron alkoxide halides (see Table 12.23) were supposed to possess the cubane-like or Ti4(OMe)i4 type structures. The Mossbauer spectroscopy indicated the presence of 2 types of iron atom coordination in Fe4Hal6(OMe)6 4MeOH and only one kind of octahedral arrangement in FeCl(OR)2 and Fe4Hal3(OMe)9. For the 2 latter products the layered structures of the Al(OH)3 type were also proposed [867, 1065]. 

A different type of compound, CutCLtCOR, where the alkoxide is 2-diethyl-aminoethanolate, has a cubane-type structure with a CU4O4 cube and tetrahedral Cu11. 

Figure 52 shows the structure of [Mn4" (HL33)(L33)(NCS)4], which can also be viewed as either a dimer-of-dimers or a severely distorted butterfly. The links between the two macrocycles are two triply bridging alkoxide donors, one from each macrocycle. The wings of the butterfly have closed to bring the two outer manganese ions quite close to one another an extension of this movement would result in formation of a cubane. 

The alkali metal hydroxides are the most basic of all hydroxides. They react with acids to form salts and with alcohols to form alkoxides. The alkoxides are oligomeric and the degree of polymerization can vary depending on the particular metal and the state of aggregation. The ferr-butoxides, MOBu, (Bu = OCMej) can be considered as an example. Crystalline (KOBu )4 has a cubane-like structure and the tetramer persists in tetrahydrofuran solution and in the gas phase. " - By contrast, (NaOBu )4 is exclusively tetrameric in thf, but is a mixture of hexamers and nonamers... 

The crystal structure of the tetranuclear complex is assembled from one [La(H2Ll)]+ and two [La(N03)2] cations, molecule [La(HLl)], and one NO3 and two bridging [/t3-OH] anions. The [La(N03)2] species are themselves bridged by the two [/ts-OH] groups and the alkoxide site (Figure 7, left). The tetranuclear core forms a rare scoop-like motif (Figure 7, right) as opposed to other more frequently observed tetranuclear cluster core structures, such as distorted cubane, tetrahedron, butterfly, or square-shaped assemblies. 

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