Highly branched species

A further, and undoubtedly increasing, class are those compounds like [ (Me3Si)3Si 2GeCl]2 which can be viewed either as a highly branched mixed element chain with 18 E elements, or as a dinuclear species with bulky group 14 ligands. 

An anion, Gc(SiMc3)3, has been isolated407 as a lithium-crown salt and shows a pyramidal structure (Ge-Si = 236.7 pm, SiGeSi = 102.6°). As expected, the angle is more acute than for R3SG ions but less so than for Ph3ELi (E = Sn, Pb) (Section II.D). 

Crystal highly disordered, large uncertainties (2.8 to 5.2) in Ge-Cl, Ge-Si, Si-Si bond lengths 

Sn-Cl = 243.3 SiSnSi = 142° SiSnCl = 99°-106° Compare Si3Ge analogue above 

W-Si = 265.2 404 SnSiSn = 100.4°-103.9° Quotes unpublished work Sn-Si = 256.9 pm in Cp2Zr(Cl) analogue Indicated, not characterized 

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Fig. 7.9. Effect on octane quality of adding 3 ml/US gal tetra-ethyl lead to pure alkane and alkene fuels. The Performance Number is a practical measure of the propensity of a fuel to knock and related to the research octane number, RON, by the empirical formula PN = 2800/(128-RON). Except for a few highly branched species, the TEL increases the the performance number of alkanes by a constant factor of 1.5. It is less effective in alkene...

Notice that, for sparsely branched polymers, most of the chains are linear, but the amount of more highly branched species increases as a 1. 

Recently, highly branched macromolecular polyamidoamine dendrimers have been prepared with Co11 bound where the metal ions have additional exchangeable coordination sites.450 These macromolecules show a capacity for catalyzing the hydrolysis of phosphate esters, presumably via intermediate bound phosphoester species. 

When the metal complexes constitute the peripheral units (Fig. lb) and/or belong to the branches (Fig. 1 c) of a dendrimer, a number of equivalent metal-based centers are present since dendrimers are usually highly symmetric species by their own nature. The metal-based centers may or may not interact, depending on distance and nature of the connector units. Multielectron redox processes can therefore be observed, whose specific patterns are related to the degree of interaction among the various units. 

In this context, much attention is currently devoted to the preparation of highly branched tree-like species, variously called cascade molecules, arborols, or dendrimers. The reasons why such compounds are interesting from a fundamental viewpoint and promising for a variety of applications have been reviewed and highlighted by several authors. Many of the dendrimers obtained so far are organic in nature. This review describes a novel family of dendrimers containing metal ions, prepared in our laboratories over the last few years. 

The interest in highly branched polynuclear metal complexes, and more generally in dendritic species, is related not so much to their size, but rather to the presence of different components. An ordered array of different components can in fact generate valuable properties, such as the presence of cavities having different size, surfaces with specific functions, gradients for photoinduced directional energy and electron transfer, and sites for multielectron transfer catalysis. Studies along these directions are underway in our laboratories. 

Hydride or methyl group shift to form the more stable carbocation species occurs during catalytic cracking. For example, methyl group shift to form a tertiary carbocation from a secondary ion species is favored. The presence of relatively high concentrations of branched species in FCC products can be explained by this mechanism. 

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