Alkoxides and their higher homologs

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. 

The same structural type of dimer involving a central four-membered ring core has been established for bulky thiolate ligands such as the 2,4,6-tris(isopropyl)-thiophenolate anion [138]. 

More complicated cases of dimeric self-assembly worth mentioning are [Ph4Si20(0LiPy)2]2, 157 [131], and [Li PhCH2C(Ph)S02CFj (THF)]2, 159 [133]. 

Rare examples of trimerized thiophenolates and selenophenolates are the supermesityl derivatives [Li(SC6H2Bu 3-2,4,6)(thf)]3, 169, and [Li(SeC6H2BuS-2,4,6)-(thf)]3, 170 [140]. The crystal structure of the sulfur derivative, for example, consists of neutral, well-separated trimeric units. The central part of the molecule is 

For alkali metal t-butoxides and trimethylsilanolates the heterocubane structure is found for the heavier alkali metals potassiiun, rubidium, and cesium. For sodium a related heterocubane derivative can also be obtained with a suitable phenolate ligand providing additional donor sites. One example is tetrameric [Na OC6H2-(CH2NMe2)2-2,6-Me-4 ]4, 172 [136], which contains tridentate pincer phenolate ligands and pentacoordinate sodium ions. 

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